Complete noob question about making wireframes
 

In a hurry with Omnigraffle - tried looking but struggling.

I need to make a wireframe diagram and cannot get my lines correct.

Any links or hints in the user manual appreciated as I can't see the wood for the trees!

I need to show top level pages connected with their sub sections like this:

[IMG]http://www.projectpier.org/files/u16/ProjectPier-0_8-Sitemap-650.gif[/IMG]

Apologies this is so low level, I'm just struggling!