I occasionly use a 236, and 237, for my measurements.
I use the following hardware for GPIB connection:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/201586
If you have only one instrument then it will plug straight into it, otherwise you'll have to get a bunch of GPIB leads, below is an example:
http://uk.rs-online.com/web/p/parallel-cable-assemblies/4544784/
The VI's I got from the internet, I do know though that to get them to work I had to visit the Keithley website and download their KIOL (keithley Input Output Layer) first. If you cant find any, then I'll try and get the ones off my computer onto here. (A different computer thats not on the web).