I am new to computerised cars and Unidiag, so I can only tell you what I have learned in an hour or so of experimenting with the programme. Please note that this only my experience with one car, A 156 2.0 JTS, one computer, and one interface cable. Your results may vary! The cable I use is a USB one which I bought from ECUFix.com who I would heartily recommend, cable arrived in New Zealand in less than a week.
To connect to the car's computer the following steps worked for me .
1. Connect interface cable between computer and car
2. Open Unidiag programme.
3. Select the |Connect| Tab at Left of Unidiag screen.
4. Select |COM4| from drop-down list below the |Connect| Tab. I am not sure if COM4 will be right for your equipment, I started at COM1 and tried them in turn until there was a connection
5. Select the appropriate definition file for your car from the drop-down listat the top Right of screen.
6. Turn ignition key to "MAR" position (Everything powered up but engine not started).
7. Click the |Start + ID| button at top left of screen.
8. When connection is made a code will appear in the "Keyword Received" area in the centre of the screen.
I found it useful to also have a screen capture programme running so I could record the information on each screen.
Once you have made a connection you can move to the |Identification| tab and view details of your ECU. I found these were already displayed with no need to click on the |Read ECU Identification| button.
The next tab is the |Diagnose| screen where you can read and clear any Diagnostic Trouble Codes (DTCs) recorded in your engine's ECU. The |Read DTC by Status| button does nothing with my car and the screen underneath helpfully explains "status not used". I don't remember now if I had to click the |Read Status of DTC| button to bring up the DTCs recorded, but clicking on the |Clear DTC's| button wiped them from the ECU.
That is as far as I have got so far, next time I feel like experimenting I am going to try reading/recording realtime data.
Best of luck. Let us know how you get on.