You do not need to wait for the categories to load in if you want to purchase EMP with your Steam wallet. As long as you can see the "Buy EMP" button in the store, you should be able to launch the Steam EMP purchase flow. You can then use the web store to purchase items if you are still having trouble with the in-game store.

My team is still working on tracking down why some people are having trouble with viewing the in-game store.