Do not get fustrated so easily, since you have this phone, try out & come back here if you have problems. Some installed it once & works, but some need a few tries. You can read this whole thread to get a better understanding. Any cab installed can't work must be uninstall.
Try this if you have not:
1. Connect Omnia to PC using the data cable on Activesync.
2. Download file onto PC from link here >>>http://www.microsoft.com/downloads/details.aspx?FamilyID=e3821449-3c6b-42f1-9fd9-0041345b3385&displaylang=en
File name - NETCFSetupv35.msi
File size - 33.3mb
Select your download speed from drop-down box
3. Double click on the downloaded file
4. Follow instructions on your PC screen
5. Continue installation on your Omnia screen
6. Exit Activesync
7. Detach data cable from Omnia & PC