Xperia X10 Sideloading Apps Problem

First off, I'm mad that AT&T has this phone locked down. Second, I'm mad that I can't upgrade to 2.1. I'm wanting to sideload an apk using the AndroidCentral Sideloading Wonder Machine but it doesn't recognise my phone. I've done everything. I do not want to root the phone. I just want to install a simple apk file without the Error message popping up saying "For security, your phone is set to block installation of applications not sourced in the Android Market." Any ways I can get around this? Please and thanks!