Place in Add-Ons folder.
OOOR get an SVN client and use that to download. If you do end up downloading an add-on but it has no mian folder (just a few folders called materials etc. with an info.txt) then create a folder and name it after the mod. It does not have to be specific due to the info.txt handling when the items are called in the game by saying which folder contains which mod.
How to use SVN after installation and restarting:
1) find URL for download and copy it (Ctrl+c or if dealing with more then one place in a temp txt file)
2) create folder and name it so you can remember what mod it is.
3) right click the folder and choose SVN checkout.
4) paste the url you found in step one in the top box and make sure there is no duplicate folder ( add-ons/phx/phx [name may vary, just make sure only 1 folder after addons\ ])
5) press OK and wait for mod to be done downloading ( may take from 30 sec. to 10 min )
6) Press OK when it is finished.
7) start garry's mod and hope you did not forget anything up.
If did correctly then the folder containing the mod should have a green circle inscribed with a check mark. (other mods that you have DL'd from a .Zip will have a purple ? and bad mods to have a red X)