My point is that given your situation (no parallel or “true” serial port available at your laptop), a “cable” will not be sufficient. The products I listed as examples all contain a microcontroller with a specific software running on there. If you cannot accept the high price, you may consider building your own “adapter”, see for example my post here: EEG Triggers via Labjack: How to do the Parallel Port DB25 connection
I am currently writing up a exhaustive documentation on this matter together with @tstenner … but it’ll take some more months until it’s out.
Hope that helps!
Stefan
PS: I don’t think sending signals from Powerlab to your laptop would work … you’ll have the same problem of missing ports that support some kind of TTL signals