When entering text for your new hub, you can simply go to your list of hubs under the "my account" tab, then higlight the specific other hub you want to link to and copy it (Control-C). Basically this will copy both the title of the hub along with the URL for the link. Then return to the new hub and simply paste (Control-V) it where you want it. I found that to be the easist way.
You can also use the link icon in the "text" box, Simply highlight the text you want to be the link, clink on the link icon in the menu bar. When the form pops up, simply enter the URL to the other hub you want to reference.
I hope this helps