Enter your anchor text you want to link to first.
Then use your mouse to highlight that anchor text.
Next click the chain icon in the editor and proceed to create the link.

When you say certain posts is there some way you can isolate them by Tags and Categories? If so then you can let the software do it for you. You can create a custom menu and add various 'items" to it such as Tags and/or Categories pages. See here http://en.support.wordpress.com/menus/