Oh, sorry, the article that I posted wasn't too clear about how to actually check permissions.
Click on the folder that you want to use. Press Command-i, which will bring up the Info window for that folder. You can also go to the File menu, and click on "Get Info" - which will give you the same result. You will see the "Sharing & Permissions" section near the bottom of that window. Unlock the settings by clicking the padlock on the bottom right corner of that window.
You'll see your user name, with (Me) (because, well, it is you!). Click the up/down arrows in the Privilege column, same line as (Me). Make sure that Read & Write are selected, (Don't touch the Privilege setting for any other lines, just your user) then you can close that window, and continue with what you were doing. That should work this time.