how to install custom worlds on mac?

@emiibelll  Are you talking about a third-party world, not one from the Store or Exchange but one hosted on another site?  If so, download the world, unzip it if necessary (so the file ends in .Sims3Pack), and place it in Documents > Electronic Arts > The Sims 3 > Downloads.  Then open the launcher's Downloads tab, check off the world, and install it.

The installer may be hidden behind the launcher itself, but you'll know it's open if you have a second plumbob icon in the Dock.  Click that to bring the installer window to the front.

For Store and Exchange worlds, the files should already land in the Downloads folder, so all you need to do is tell the launcher to install them.  If nothing happens when you try, close the launcher and open it again.

when i try and install nothing happens?

@emiibelll  Again, you may need to close the launcher and open it again.

If that doesn't help, try installing a world into a new user folder.  Move the entire Sims 3 folder out of Documents > Electronic Arts and onto your desktop, open the launcher to spawn a clean folder, move the .Sims3Pack file to Downloads inside the new folder, and try to install it.  (Close the launcher and reopen it if it doesn't work the first time.)  This will temporarily remove your saves and other content, but that's fine, we can address that later.  For now, just let me know whether the world works at all.