Macbook Air wont recognise my external microphone

The issue is not the physical port size, it is the port's function. The audio port on this MBA is an audio out port for things like headphones, not an audio input port for microphones.

Ref: https://support.apple.com/kb/SP714?viewlocale=en_US&locale=en_US

eden168 wrote:

I have a 2015 Macbook Air. I recently purchased a Neewer-700 condenser mic, I have the XLR cord connected from my mic to the input on my phantom power and the output XLR from phantom power to the Audio jack in my macbook.

... because the audio port on the 2015 MacBook Air is a headphone jack?

That adapter appears to be used for headphones that also have a built-in microphone. Sorry, but I have no way to verify, whether or not, it will work for what you are intending it for. You may have to just give it a try and let us know.