This took me way too long to figure out. And to be honest, the game didn't make much sense because the cards and upgrades that I bought didn't appear.
I thought it was something to do with not reaching a certain level but now it looks like a bug with Firefox, which is the browser that I'm using.
See the 'Click Here' button in the region I circled? Once you click that, the cards will be updated and you can equip them as needed.
Previously, my cards showed up in my collection but I was unable to equip or use any of them.
Hope this tip helps any new players out there struggling to understand how any of this works!
