Well actually you probably could have undone it.
being and admin isnt everything! there is also the root
but root aside, you can in the finder use the 'Get Info' option on a folder, in the window that pops up then you have a 'Sharing & Permissions' section. there you can see whether or not you have access to write/read to that folder. As admin you can change the permissions, so if you dont see yourself (or the group admin) in that list with 'read & write' privillage, you need to press the lock under that section to unlock it, so you can make changes, it will ask for your password, enter it, then you can change the permissions there. Simply press the +, add yourself, with read & write permission, then use the gear button and choose Apply to enclosed folders/files (so you also have access to the files in that folder).. that should do it..
(if that still fails there is also the application chmod to change permissions (or the same terminal command, but then you dont get the nice interface), but be careful not to mess things up
)