I want to thank everyone who reached out regarding difficulties installing our Foundry modules, and follow up with some additional guidelines for installing the content. We are working with Foundry to make the process smoother moving forward, but in the meantime, following these steps should allow you to access your modules. Foundry can be quite finnicky, and it can be easy to overlook a step.
Note: These steps are applicable for the Foundry desktop application, not for online hosting services such as Forge. We are in the process of resolving any existing issues on Forge as well.
Step 1: Go to your Foundry account
Go to foundryvtt.com and click on your name to go to your user profile.
Step 2: Go to your Purchased Content tab
From your user profile, scroll down and click on Purchased Content.
Step 3: Enter your Backerkit Access Key
Copy your activation key from Backerkit into the Content Key field, then click "Activate Content."
Step 4: Copy Manifest URL
Scroll down to your activated content and click on the name of the module. From there, click on the Manifest URL.
In the manifest URL, the fifth line is labeled "manifest". Copy the entire URL within the quotation marks (be sure not to include the quotation marks in what you copy!). The manifest URL that you copy should look something like https://path/to/mythcraft/module.json (this is an example. The actual URL will be longer. This will not work if you have not activated your content first.)
Step 5: Install Module
Open up your Foundry VTT desktop app, go to Add-On Modules, and click Install Module. At the bottom of the Install Module window, past the manifest url into the appropriate field. Click Install.
Step 6: Activate your Module in a MythCraft Game
Open a MythCraft game on Foundry, go to settings, then go to Module Management. Activate the modules, then restart the game.
Step 7: Copy Compendium Content into your Game
Go to Compendium Packs, open each compendium, and copy them into the appropriate Foundry tabs. For example, all creatures should be copied into your Actors tab, the rulebook text will go into the Journals tab, and most of the rest of the content will go in the Items tab.
Step 8: Play!
With the compendium content distributed into your active game tabs, you should be ready to get to mythcrafting!
Thank you for your Patience
While not particularly straightforward, this sequence of steps should get you playing MythCraft on Foundry while we continue to iron out the process. Thank you all for providing feedback that helps us give you the smoothest possible gaming experience!