Hi
If you want a refund then you need to contact EA customer services.
However before you do that why don't you have another go. You say you downloaded the game so I'm assuming you are using Origin rather than the disc version. Is it just the base game that you have or expansions as well. I will put copious instructions to help you.
First thing to do is to remove all traces of the game and of Origin (see 'clean uninstall' instructions below). Then try the download again (see 'using Origin' instructions below). Once you have the base game installed, patch it up to date (if the update tab of the launcher doens't work, download the Mac manual superpatch from the Mac forum of thesims3.com website).
If you can get it installed then you should check the additional information on playing the game on the Mac operating system. The 'unknown error' is unfortunately very common but if you follow the advice regarding purging RAM etc, you might be able to get it working.
Caspin
Clean uninstall
When you uninstall TS3 games on your Mac using the uninstaller, the game leaves behind stray files and isn't completely removed from your computer. These files may conflict with future installations. It is best to completely remove them. BluebellFlora's website provides a guide on how to do this:
http://bluebellflora.com/the-sims-3-on-a-mac/how-to-do-a-clean-uninstall-and-reinstall-on-a-mac/
Uninstall and delete all trace of Origin as well.
Using Origin
The first thing to check is that your Mac is sufficiently up-to-date to be able to run the Origin client. It requires at least OS X 10.6.8 with an Intel Core 2 Duo processor or better.
To download and install games through Origin:
First install the Origin client for Mac and make sure it is up-to-date. The latest version can be downloaded from here: http://www.dm.origin.com/mac/download
Additional advice on how to download games (thank you Tremayne for this tip):
When downloading from Origin, do NOT use the first install icon. Move your cursor over the game cover and click on the information (letter i) and then click on the install button from there. For some reason, downloads from Origin frequently become corrupted. Using the i reduces the chance of your download being corrupted.
Are you running anti-virus software or any clean-up applications? If so, disable these, as they might be deleting files that Origin needs!
Storing Origin download installer files:
If you are going to change your computer, or just need to uninstall/reinstall your game, you may wish to set your preferences in Origin to save your installers. This will avoid you needing to re-download the games again.
In Origin, go to Preferences. Choose “Advanced” and look in the section entitled “Game Installers”. Tick the box to “Keep game installers after games have been installed”. Choose a location in which you would like the files to be saved (for example, create a folder in Documents ready to receive these files).
Additional information for playing TS3 on Mac
Unfortunately a lot of TS3 players encounter problems when playing the game on their Mac. These can include crashing and freezing. A lot of these issues can be avoided or at least improved by the following measures.
Firstly, manually reset your RAM while playing. See BluebellFlora's guide on how to do this: http://bluebellflora.com/the-sims-3-on-a-mac/resetting-ram-during-gameplay/
I restart my computer before I play TS3. The memory used as seen in Activity Monitor is at about 3GB after the restart. If I open the Launcher it goes up to around 11 GB (that's all I've done - just opened the Launcher!). At this point I reset the RAM. Then I hit play and get through the opening sequence. I then press cmd-return to put the game in windowed mode from time to time while playing (and always before saving or using CAS). That way I can keep an eye on the memory and reset RAM if necessary. Being in windowed mode also allows you to close the game using force quit if it freezes up - it's better to crash the game than your whole computer.
Second tip is to save frequently. I save at least once per sim day. Use the option to Save or Save As, rather than the combined Save and Quit.
Make sure you are patched up to date. The 1.63 patch included fixes for Mac simmers that did actually improve the stability of the game on our operating systems, so it is well worth updating.
For general advice, information and tips on how to play TS3 on a Mac, read BluebellFlora's guide here:
http://answers.ea.com/t5/The-Sims-3/Links-to-Mac-help-threads-and-guides/m-p/749664#U749664