I've been asked a few times now why a particular community lot cannot be placed anywhere. 

Community spaces are a bit scarce in Sims neighbourhoods, and if you have a community lot you want to place, you need to convert a residential space into a community space.  Unlike Sims 2, it is very simple to do in Sims 3.

1. Start The Sims

2. From the menu, select Edit Town

3. Click on the lot or space you wish to convert.  It needs to be the same size or bigger than the lot you want to place.

4. Click the icon that looks like half a house and half a bench (shown by the red arrow).  This toggles your lot between residential and community.

And that's it!  Some community lots play equally well as residential lots, so you can convert those too if you wish.