If you are an active member of MuseThemes, simply:
You will also see a button labelled "Add to Cart." This doesn't mean there is anything wrong with your account; ignore this button.
You may have more than one account setup with MuseThemes, but only one account will be tied to your active subscription. This is the one you should login and download with.
If you are not a MuseThemes member (you haven't paid for unlimited downloads), you can purchase single templates; simply:
You click the "Download" button, but are rerouted to a page asking you to join
This means you are either not yet a member and need to pay for the single template or you haven't logged into your account. You can see your login status by referring to the upper-right hand corner; it should say "Signed in as..."