I'll have a look into the problem ASAP, thanks for the report.
As for requirements, they should all be automatically detected and installed by the installer, i.e. XNA, C# 2008, all necessary frameworks etc.
Plugins are not yet a feature we have implemented, but you can make games without the need for them :)
Tutorials can be found at
http://www.youtube.com/user/ExpressGameMaker, as these are the easiest to look at and follow for now.
Good luck with EGM!
EDIT:
I just managed to sign up with both a yahoo and hotmail account without receiving errors, are you certain you typed the email correctly?
Could you try to register again? (Use the name "TestEmailReg" so I know which one to delete later.)
This time use a hotmail or yahoo email address (or any other that didnt work before) - and print screen the error occurring to post here?
Some tips - did you try moving to the next box after the error message came up? This validates the text box. Also, if it continues to fail, can you try filling everything in with useless information and pressing register anyway? Again, i'd like to know what happens.
Thanks!