I just re-installed the sims 3 and I haven't played it in a while. I can't remember how to place downloaded lots in my game. My mind is a complete blank! Help!

Place lots from Edit Town mode, but firstly you need to check lot size and designation to make sure your new lot will be placeable where you want it.

Or in case an utter new person as I am reads this thread in hope of enlightenment, first you use the thing that looks like a grid and pencil to place a space at least as large as the lot you want somehwere on the map, and then you select that empty lot and use the menu that comes up to classify the lot to precisely the sort of thing it is going to be. Only then can you place your prebuilt lot or build your own lot. That was kind of difficult to comprehend at first as the game gives little guidance.