Unable to open files copied over from other macs

I've just bought a new macbook and copied over a series of files from my older desktop (imac) and laptop (macbook pro) onto external harddrives. When I try to open these files (images, videos), I get this prompt:

"You do not have permission to open the document..." "Contact your computer or network administrator for assistance".

I followed discussion board advice and tried to change the permission settings in each file by going into file's 'get info' window--clicked on the locked button--unlocked it--changed the permissions.

However, the changes do not take effect, and the file remains to be inaccessible. I don't even get a preview. Could it be that the password needed is from the older machines? I don't remember that password and have no idea what to do...

HALP!