You will want to first use your mouse to click the keys on the virtual keyboard for the virtual instrument that you are loading. This way you can test if the instrument itself is producing audio. If it is not, please re-install the Soundset or 3rd-party VSTi that you might be using. Once you have verified that your virtual instrument is working, open the MIDI monitor at the bottom left-hand corner of the screen and verify that you are receiving MIDI messages from your Akai. If you are, you will want to open the Inspector window by clicking the icon of the letter "i" at the top left-hand corner of the screen and then verify that you have the correct input selected for the device. You may also want to take a look at the "Control" drop-down in the virtual instrument that you are working with.
If you are not receiving MIDI messages, you will want to go into your External Devices window and verify that you have selected the correct MIDI port for "Send to" and "Receive From." If the ports do not show up, you will want to try using a different USB port or contact the manufacturer of the device.