I just did it and it is a little goofy.
Here is what I did:
1) Start the data capture program for the Alt15K. The Stratologger and Pnut programs do not work for the 15K.
2) Connect the altimeter to the cable and plug the cable into the computer.
3) Turn on the altimeter.
4) Quickly select the Altimeter tab in the capture program
5) Quickly select the CommPort option.
6) It will show 4 ports and my screen shows none are present, but the upper one can be selected. Select it. I suggest you repeat steps 2 to 6 if there is no commport active. I had to redo these steps several times, but my USB cable may be the problem.
7) Select Data tab and then Acquire. Just ignore error messages if present by clicking OK option.
8) It then gives me the option to download one of the last three flights.