iMac 21.5" Headset Woes

[SilencedBacon]

Here are the products I am using;

http://www.amazon.co.uk/gp/product/B00275C2L6/ref=oh_details_o05_s00_i00

http://www.amazon.co.uk/gp/product/B005M19I0K/ref=oh_details_o04_s00_i00

I bought the headset before realising the iMac doesn't have the pink jack for microphones like all PCs do (my first iMac) so I got the second product listed, an external sound card which works via USB. My problem is the headset sounds works fine but the microphone does not work

I have tried playing with MIDI and System Preferences, I got it to work once but I do not know how and I cannot recreate it. I come here for potential help.

Thank You