I have recently installed a memory upgrade to my MacBook. My “About This Mac” recognizes the upgrade, but my computer now says there is no memory at all and that all files are full. Does anyone know what happened?
try to remove files to external hard drive
What you've described is not possible. If you installed more RAM and you say the Mac recognizes it's there, then it is indeed there. However, you seem to be confusing memory (RAM) with hard disk space. If you truly have no free disk space left, that would be the source of your problem.
Mac OS X requires that have at least 10% of the size of your hard drive free, or bad things will happen, such as files spontaneously combusting. So you need to start deleting all the crap you don't need, and move stuff you do need but rarely use or look at to an external storage medium, ASAP.