The site map is currently a nightmare to access with anything other than a mouse. Because you can't use keyboard shortcuts (such as +/-/space to toggle branches, and arrow keys to move around the site map) it's impossible to use if you don't have a mouse.

The following link provides some additional inforamation:

http://www.rnib.org.uk/xpedio/groups/public/documents/PublicWebsite/public_sitemaps.hcsp