For me, I had to take the SD Card out of the GPS, and insert it in the card reader on the PC for HomePort to detect and load the map... I'm not sure this is the supposed way to go, but based on other posts I believe it is.
Pratzert, are you saying that HP can read your map directly from the GPS through the USB connection? It doesn't work for me...
Note: I use a 76CSx and Bluechart g2 map.