App Store won't let me sign in
There is but one problem. When asked to log into App Store, in my case to download some updates for my Macbook, the username and password dialog box already have a username written in – and it's greyed out, so I can't change it.
Now here's the twist: It's not my username that is greyed out.
In fact, after trying several times, I realise the dialog box only gives me two options, switching between two e-mail addresses I don't own. Surprisingly enough though, I know who owns one of the e-mail addresses, and it's a close friend of mine. I'm a 100 % positive this isn't something he has done on purpose. But the problem still remains, and none of us know why.
So I got an idea, that if my friend could log into his account, and then log out afterwards, the App Store dialog box would allow me to write my own username. I tried that, and it worked great – until I was rejected in the same way as before, after having rebooted the Macbook.
I have searched many apple forums for the answer to this, but it seems I am alone with this. Any help or advice would be greatly appreciated!
Here's a photo of the dialog box, with a greyed out apple ID field.
MacBook Pro, OS X Mavericks (10.9.1), App Store