''Click on Advanced then change the com port in the drop down list''
After doing this the screen that pops up is in the general tab (tabs at the top). Click on the port setting tab then the drop down list should be there. Select the port you want to use, then select that port on vagcom.
If this doesn't work try changing the interface to a different port on the drop down list as the original may already be being used by something else and casuing a clash.
I was struggling with mine, and this way worked for me.