3.5mm mic input not working, MacBook Pro (2009)

My 3.5mm mic input on my MacBook Pro is not working. I am using an external Antlion Modmic with a 3.5mm plug. I have tested the external mic on an iPhone 6s Plus and it is working. When I plug into the input on the MacBook the meter on the system drivers show activity, but when I speak into the mic there is no input. Internal mic is working as normal.

System info;

MacBook Pro (15-inch, 2.53GHz, Mid 2009)

OSX Yosemite 10.10.5

2.53 GHz Intel Core 2 Duo