Simple encryption software for the Mac
You can do it with a .dmg.


Open Disk utility
Select New Image or File > New > Blank disk image
Choose the name, where to put it, the size (it will be pre-allocated)
Choose an encryption
Leave the other settings by default
Choose a password


Now, all you have to do when you want to access that folder, is open the .dmg file.

And it's free.

I found an app that took care of all of that in 2-3 easy steps but can't seem to find it again. You might have more luck.