Help downloaded stuff on the exchange and brought things cant find them.

Once you successfully download an item from the store, it should land in your Downloads folder within your Sims 3 folder in Documents.  You should then be able to see it in the launcher under Downloads; you have to install it from there.  When installed, the restaurant will appear in-game in your list of community lots along with any building added by the expansion packs you own.  You have to go into Edit Town to see this tab, which will be next to the tabs for premade families and houses.

When you're in Edit Town, you can select any premade building and choose "place" to set it down on an unoccupied lot.  Keep in mind that the empty lot must be at least as big as the building you'd like to place.  If you want to use a residential lot, you'll have to change the zoning first, which you can do by clicking on the empty lot.  The placement might also not be available if there is any object or sim currently standing there; you can see whether this is the case in live mode.

Thank you and then once it is placed can my sim start to make its own business ? sorry first time ever i have got something from the sims website

Once the business or venue is placed in town, your household can "become a business partner" (if it's a rabbithole business, like the bistro or grocery store) or just buy it outright.  To buy outright, you can visit the lot or handle it over the phone, but to become a partner, you have to click on and travel to the business itself.

You can upgrade venues (but not rabbitholes) to make them pay out more money; when you click on one, it will list the requirements for an upgrade, up to three stars.  But without mods, you can't do more than buy and sell them and collect the money, and in the case of rabbitholes, fire employees if you feel the urge.  As far as those mods go, I don't use them, so I can't help you there.

https://store.thesims3.com/setsProductDetails.html?scategoryId=13570&productId=OFB-SIM3:67573&section=UpSell that is the one i brought do you think id be able to run it as my own business sounds a bit boring just buying and selling them lol

Thanks for all your help

I think the point of this restaurant is to be able to watch your sims eat there, instead of the usual rabbithole experience.  It doesn't look like there are any other special functions.  But you can google what people have done with it if you're interested.