Here are a couple of ways to add a link:
1. Add a link capsule - Copy the URL of the hub you want to link to. Click "edit" on the link capsule. It will show "Add links by: URL or Search." Choose URL and paste the URL you just copied. Click save.
2. Highlight a word or phrase you want your visitors to click on. For example, your hub is about shoes and you found another hub on red shoes. Highlight "shoes" or "red shoes" in your hub. Click link or the chain link on the right top of your capsule. Paste the URL on the space provided. Click "Insert" to the right of the URL.
If you need more information, go to this forum thread.