It’ll be with Option + ⌘ + I on Mac 
Hmm, alright I don’t own that one so it’s a bit tricky to check.
Could you please check in your Foundry server logs from The Forge to see if there are any error messages there, which might explain why the install is failing?
For premium modules, we forward the install request to the Foundry server.
As an alternative install method, you can install the package on your local Foundry server by following the steps given from Paizo:
This purchase will grant you an activation code which can be activated through your Foundry VTT account page at Home | Foundry Virtual Tabletop and access to the PDF version of the same content (both downloadable from your My Downloads page on paizo.com). Afterwards you will be able to install the “Sky King’s Tomb – Mantle of Gold” module on the Foundry Virtual Tabletop setup screen. To learn more about activating and installing premium content in Foundry Virtual Tabletop, visit Premium Content | Foundry Virtual Tabletop.
And after that, you can use the Import Wizard to import the package to your Forge account.