I just looked at the MS Webmatrix application. May I ask why you're using it, and is it local?
Not knowing how it works exactly, it's hard to say why you can't log in. Who knows what's up with their installer. You could have an installation with no user (or password) in the MODX user table, you could have incorrectly configured config files, your DB user could have incorrect permissions, your user could simply not have a role specified (check the user table), etc. It could be anything. There are a number of threads here you could check as well. Search for something like "new install can't log in" or something similar (get creative, like also "unable to log in").
Outside of working through the many possibilities of solutions others have found for various servers...
Can we help you install MODX either locally on XAMPP, or on your server directly? That's really the best way to install MODX. These scripts are not the most reliable for installing MODX. There's been a whole host of different kinds of install problems reported after using an installer script for MODX. Usually the easiest solution is to uninstall and then do it the proper way, manual install. It's actually very quite straightforward and quick. See
http://rtfm.modx.com/display/revolution20/Server+Requirements and
http://rtfm.modx.com/display/revolution20/Installation for the specifics.
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