Sweet, you're all good to go then :)
The error you got is not anything to worry about, just to say that one of the install methods for one of the dependencies failed - however, it would have moved on to a install method that does work on your system, as otherwise it the error would have been fatal (in regards to the program that is ;)). Generally, unless something crashes docpad (which rarely happens), it isn't anything to worry about :) But thanks for letting me know, I'll be sure to document this a bit more.
Sure, so DocPad can either be used as a replacement for
compass.app, or as a complete way to write your website. Nearly everyone uses it as the latter, as DocPad also offers layouts, partials, and meta data as well as the standard pre-processor support. You can even accomplish things like paging, and search, and even form submission etc with DocPad too if you wanted to go that far!
So glad that all helped, and be sure to remember we're always here to help :)