Will iRig connect my electronic guitar to my MacBook Pro?
If your Macbook Pro has the single 3.5mm jack that can take input and output then iRig should work just fine for you.
Someone asked a similar question here. If you're looking for something ultra cheap you could always get a 1/4" to 3.5mm adaptor to plug your guitar into your Mac. I did this for a very long time until I could afford higher quality hardware. These things run as low as $2 at some places, here is one for example. Hope this helps!