To find out whether you've got an ISA PNP midi port, check with the Devices preference application. In the "System Devices" section, you would typically find a device of type 'Sound'. Looking at its resources, you would see that it occupies a single IO space of length 2, typically at 0x290, 0x300 or 0x330 as well as one IRQ, typically 5 or 10.

On the "Info" tab, you should have a line saying 'Vendor: PNP Product: b00 Revision: 6'. This is what the driver looks for.

If you have an ISA device, BeOS cannot automatically identify it. You will have to find out about the resources it is using (in Windows or from the manual) and set up a so called jumpered device. Instructions on how to do this are included in the package. ISA MPU401 devices exist either as simple older boards for connecting an external synthesizer or as interfacing devices on synthesizer cards.