Confusion about Apple ID on newly purchased MacBook Pro
I have an iPad and iPhone and therefore an Apple ID. I just bought a MacBook Pro. When I set it up I think I was re-assigned a me.com user name for my Apple ID. What happened? The Mac wouldn't accept my actual Apple ID, but said that it had "found my user name, " @me.com. I continued with set up. Right now I can use my original Apple ID to buy a new app, but not to update the existing apps. I get an error message that says "Keynote (Numbers and Pages) cannot be updated because it was refunded or purchased with a different Apple ID." I actually tried to change to the @me.com Apple ID, but that was a useless move. Not sure what to do. Ultimately I'd like one (the original) Apple ID that works for everything. Aren't we only supposed to have one? Appreciate any help. This is my first Mac. There's a steep learning curve!
MacBook