It's usually pretty straight forward, however there can be two problems.
1. Some BT Dongles don't have the correct Profile for using ActiveSync, the only fix for that is to get a Dongle that does.
2. The Com port can get confused.
Basically,
Turn BT On and make Discoverable on your PC
Turn BT On and make Device Visible on your X51v, in the Mode Tab of BT.
Then in the Devices tab tap "Add new device" it should then search for and find your Dongle
follow the relevant steps to pair it, adding a passcode if necessary.
It should then show you services that are available with your Dongle,
One of them should be ActiveSync with a tic box next to it.
If it is there Tic it and OK your way out. (If it's not there then the Dongle doesn't support ActiveSync.)
Then open ActiveSync on your Axim, Tap Menu and select "connect via Bluetooth"
It should connect.
If it doesn't work, then you may need to change the com port settings on the PC
In AS on PC, go file/Connection settings
third Tic box down, Allow connections............. should be ticked
then in the drop down menu you can try different com ports, it sometimes takes a bit of trial and error to get the right one.
See how you go with that, If not let us know.
Edit: Here is a link to a
BT Guide, but it is a bit old an may not have the set up for WM5 devices.