I have heard that alot of users (especially AT&T users) cannot sideload apps on their Xperia, here is the solution.
The Sideload Wonder Machine?
I am unsure if it works as I don't really need it, but it was designed for people with Android Phones that do not allow them to sideload apps
You do not need to root your phone, all you have to do is install the software to your computer, enable USB Debugging, and make sure the USB is on 'charge only'
Then in the software, choose the .apk file that you want to install via your phone, then the software will install the app on your phone via the USB cable, so you don't have to install on your phone, because the software on your computer installs it for you
Like I said, I do not know if this works, but I've heard some positive things about it Hope I helped
More info and Download link
The Android Central Sideload Wonder Machine - Android Forums
The Sideload Wonder Machine?
I am unsure if it works as I don't really need it, but it was designed for people with Android Phones that do not allow them to sideload apps
You do not need to root your phone, all you have to do is install the software to your computer, enable USB Debugging, and make sure the USB is on 'charge only'
Then in the software, choose the .apk file that you want to install via your phone, then the software will install the app on your phone via the USB cable, so you don't have to install on your phone, because the software on your computer installs it for you
Like I said, I do not know if this works, but I've heard some positive things about it Hope I helped
More info and Download link
The Android Central Sideload Wonder Machine - Android Forums