Hi Christian,
Did you follow the "how to" guide on Unidiag in the Running Reports and Technical Fixes section of the forum? Have a look at that first but if you still can't get it working, a number of things could be causing the problem:
Have a look at the com port that the driver has chosen for the interface. From memory, this needs to be less than com8, you can change it through control panel using the instructions linked in the guide.
If that's okay, check that you have a good connection between the ISO adaptor and the positive terminal on the battery. If you have a series 1.5 car with a non-standard Protocol but a 16 pin OBD connector under the dash, you don't need a separate positive connection.
Next, if a series 1 car check you are connecting to the correct ISO connector! On a RHD car it's behind the carpet on the driver's side of the car and it's grey in colour with a cap covering the end. I don't know about a LHD car but it'll be shown on the Eurospares website if you look up the ECU or in the service manual.
Finally, an obvious one but make sure you turn on the ignition before you try to connect.
Hope this helps, any more problems PM me and I'll try to help further.
Regards,
Steve