In order to download/install the software, you need to have purchased the SELF-HOSTED version instead of IPS's hosted version. Specifically you will want to have purchased:
https://invisioncommunity.com/buy/self-hosted/
I assume you have purchased the wrong service. If that's the case, the staff will need to help you fix this. (I'm just another customer!)
You can reach the Accounts and Billing team by using this form:
https://invisioncommunity.com/contact-us/
Once another member of the staff see this thread, they can also create a ticket for you as well with that team.
After you have everything squared away... most 3rd party plugins/resources are purchased via the marketplace found in your new site's AdminCP.