just like you would link to any other site. I like doing it in a text-box module. Just write a phrase - or just the target hub's title, then above, in the toolbar, towards the right hand side, there is a little icon that looks like a link-chain. Highlight the text you want to be a hyperlink, then click that icon. An edit box will come up where you can enter the target address. Get that directly by visiting the other hub and copy paste the address. Should be easy from there.