Can't select the link button in a post

Hi there,
I am new to WordPress, but have got the site up and am adding content.
I am trying to include a link to allow members to download a document/resource within a post.
I am trying to do this in Visual mode, but the link and unlink buttons are not ‘highlighted’ so I can’t select it to add the url to the resource…
I am on a tight deadline – Please help!
thanks,
sm