Kwicky and Postdog have basically got it covered. The important thing to remember is that web addresses will automatically 'hyperlink' (i.e. appear in red and be 'clickable') if you give the
full URL,
including the 'http' bit.
For example, this 'short' URL won't hyperlink:
www.google.co.uk
but the 'full' URL will:
http://www.google.co.uk
You can simply type out the full URL but it's far easier to use Kwicky's 'copy and paste' technique. This is simplest if you've got two tabs (or two windows) open in your browser. (One for AB and one for the page you want to link to). With the page you want to link to displayed, right-click in the address bar and select 'copy'. Then switch to the AB tab, place the cursor where you want the link to appear in your post, right-click and select 'paste'. (As Postdog states, ensure that you leave a space after the link, otherwise the next bit of text will get added onto the end of the link).
Some people like to take things one step further and post links like
this . I can't be bothered with it, since it involves entering HTML text into the post but, if you want to know how it's done, see under 'Posting guidelines', here:
http://www.theanswerbank.co.uk/ABTutorial.html
Chris