Memory map recognises gps's, you have to start from that position! 😉
It sounds like your computer does not recognise the gps device. 3 likely problems will cause this. Firstly, the device driver might be installed wrongly. De-install and reinstall the device driver first. Secondly, are you using the correct type of cable? The cable you use might need to use could be a specific one – this is certainly the case with my geko 201. Thirdly, if you're sure you've got the right cable, there could still be an issue with the CABLE DRIVER – have you definitely loaded this software correctly too? De-install and re-install this driver just to be sure.
Assuming you've cleared the 3 questions above, and your computer definitely recognises the cable and gps, then you can start messing with the gps settings in memory map. These are pretty straight forward to be fair, and there's only a few fields to fill in. You need to choose the type of device ("garmin" from the drop down list). Your GPS manual should tell you whether to then use the model setting as "serial" or "usb". But if unsure try each. You need to input the licence key and serial number, and select the appropriate port.
The other thing to try is swap which usb port you are using and re-try?