Welcome to the forums, Wolfgang.
If you have the 8100 and not the 8110 or 8120, follow these steps to the letter:
http://www.blackberryforums.com/gene...a-english.html
Don't use DM, but use Loader.exe.
Also, make sure you're connected directly to a USB port on the rear of the PC, not a USB hub.
If that doesn't work, then try following these steps:
What to do next if Jl Cmder will not connect and wipe - BlackBerryFAQ