You need to edit the file and change that line. ADB is the way to go, search for a tutorial to upload and download the files from your phone via ADB if you never had done it previously.
Hello,
I think that there is a little problem with the build.prop from the last weeks on MIUI who makes impossible to download protected apps from the Market since they appear as not compatible with the device.
Some examples of this protected "not-compatible" apps are...