My GM just installed it. The rules come up under Journal and I can see the Harrow Deck in Stacked Cards. But when I right click the deck in Stacked Cards, like instructed, there is no option to do a New Reading.
Heya 
Just to confirm, make sure you’re right clicking the Harrow Deck itself and not the folder it’s in
I believe that the Paizo devs included a new macro into your Macro directory (or possibly into a macro compendium) automatically, which you can also use to start the reading
If that still doesn’t work, it might be possible that something else is interfering with the deck opening, such as another module. I recommend opening the dev tools (F12) console tab before right clicking the Harrow Deck deck itself, and checking to see if there are any error messages being logged out there that might explain what’s going wrong.
Hope that helps! If you still run into the issue, please post a screenshot of your dev tools (F12) console tab after right clicking the Harrow Deck deck!
My GM was able to start a reading. Is the issue that I’m just a player? I thought, players could do readings if the GM gave permission.
Does the GM have to do something to get the Macro to show for the players?
It’s very likely that it’s a GM / Player permission issue then, yeah. But in that case, since this is a problem with the pf2e Harrow Deck module specifically rather than a general problem with Forge or Foundry, I’d recommend asking about it on the Pathfinder 2e FoundryVTT Community Server 's #pf2e-modules channel 
I’m afraid I don’t personally own the module which makes it difficult to troubleshoot from the outside. One of the module devs or pf2e-related devs might be able to provide more concrete advice and instruction here.
Oh, and yes, the GM needs to share the macro with the players by editing its permissions