Can a Macbook 4,1 use 64bit architecture? I have a Macbook 4,1 running Mac OS X 10.7, and I need to install the MESA program, that only works on 64bit machines. Running uname tells me that my mac is working in 32bit mode, and a quick search online told me that it should be able to run as 64bit, which is what I need.

$: uname -m
i386


Following Apple's advice, I tried to get my computer to boot up again in 64bit mode, and verified it with uname again, to discover I'd failed. So, my question is, can my macbook work in 64bit mode?
If you simply have software that needs to run as 64bit then yes it is compatible, if you have software that needs a 64bit kernel (rare) then no it is not compatible. From looking at the website where it shows system requirements it shows it needs a 64bit capable CPU, which is what your MacBook has, same CPU as my iMac. As you can see below on my Late 2006 iMac with 2GHz Core 2 Duo running 10.7.5 with a 32bit kernel and all 64bit processes.