Well,
I just did the smart thing and popped my old reg keys into v1.4, and sure enough, I saw the same issue. Here's what you should do;
-Under the MIDI menu, click MIDI PORT MAP.
-This should give you a list of assigned ports, some of which will say unassigned, and a dropdown for assigning. You should go ahead and click one that says unassigned.
-Now click on the dropdown at the bottom of this dialog, and find your yamaha synth card. Click on this, and the unassigned should now change to read your yamaha device.
Those steps are just to insure that your device is available to med. The real discovery I just made is next;
-Go to the MIDI PATCH NAMES SETUP dialog under the MIDI menu.
-In the area where it reads Device 1/1 click the DELETE button. It should now read Device 0/0.
-Now you should Click INSERT NEW or APPEND NEW to create a brand new device.
-Strangely enough, the device automatically created with a new mod in v1.4 does not allow you to set a device port, but the new one that you added should.
-Go ahead and choose your yamaha port.
-Proceed with the other steps for setting up your settings and instruments.
This should take care of your problem, I hope. This seems to be a difference between 1.4 and 1.5, so I guess there were some nice improvements
Hope that helps!