connect the phone to computer
run DM 5.0.1 press application loader then press update software, u will see a list of all the updates as it shows OS 5.0
STOP HERE, dont select anything, just keep DM open

go to C:/ common files / research in motion / apploader
open apploader.exe
press next, and see if it detects it OS 5.0

this worked for me, it was probably some glitch in the system

if it still doesnt detect, KEEP BOTH those windows open
download shrink-a-os (google it), disconnect the phone, reconnect the phone, and run apploader from shrink-a-os and press next
you will have a total of 3 windows open by now (DM, apploader from common files, and apploader from shrink-a-os
see if it detects OS 5.0, if u see OS 5.0 ready to load with all those check boxes u did it right.