Can't get a working midi set up

Hi, I have an imac running yosemite 10.10.3 and Garageband. I want to connect a Roland PC-300 midi keyboard to my iMac. I didn't get it to work via USB so I dug up my Edirol UA-20 USB Audio / midi Interface, but still I don't get a signal from the keyboard. Checking the Roland website learned me that the PC-300 is no longer supported. On a forum I read that using a midi interface should work. Unfortunately that forum is closed so I cannot get help there anymore.

My set up:

- Midi output from the keyboard is going into the midi input of the UA-20.

- The USB connection from the UA-20 is connected to the iMac.

- The UA-20 is recognized by the iMac. Have set the audio input to the UA-20 and the output to system (using the imacs' speakers).

When using the on screen keyboard it works, but for some reason the signal of the midi keyboard doesn't arrive in garageband. Does anyone have any clue on how to get this work? Or is the hardware to old already...

Thnx!