Q: How do I pair the apple remote with my macbook air 2011?

Assuming that the iMac actually has an infra-red receiver:

How to Pair your Remote to your Mac:

https://support.apple.com/en-us/HT201602

Presumably it does not have a superdrive either, so if it can't play CDs or DVDs, why have a remote?

Fair point! Apple want you to use AppleTV now, then you can use the remote of your TV.

The MacBook Air has not IR reception capabilities. If you want to use an IR remote, you need to be using a MacBook Pro.

You can buy a Bluetooth remote, and use it for certain functions such as slide advance and some other limited functions, but it is unlikley to duplicate that of the Apple Remote.