When trying to insert a link onto a word in a site page on SharePoint 2010, you should see this come up :
But if you’re clicking on Insert Link like a crazy person and nothing is happening, try one of two things.
First hit Save and Close and then Edit again.
Or if that doesn’t work; simply delete the text and retype it, (sometimes you need to enter and type on the line below). Then select the text and click Insert Link again.
Works every time.