I can't speak for other systems though can't imagine it'd be any different, but here it is on PS4. It's the bottom right tab that you'll find available whilst on all of the menu screens (Konquer, Fight, Kustomize and Learn) until you actually enter a tower-select or character-select screen.

There should be a button prompt next to the Store (here on the PS4 you can see it's to press 'R2' - the second trigger on top of the controller) and the tab will then open overlaying your screen once simply pressed accordingly. Here's a visual guide to help you:

Hope that helps!

As you can also see, it also offers a prompt on the bottom left of the screen to press the opposite L2 button (or whatever button it may be on Switch, Xbox or PC) where your Kombat Kard (your personalised online profile picture and background, as seen in the first picture I've got Reptile with the Wu Shi Academy background) can also be edited.

Words of advice though if you're new to the game; the Krystals are a hefty grind and once you've done the whole Gauntlet, emptied the Krypt and stopped levelling up often, you'll only really be getting a slow-drip feed from either Kombat League or from the top 10% of Race Against Time each week (which currently has MK2 Jade as its top 10% rewards, so get completing those Towers of Time before it runs out if you've not got her!). You can buy them with real money but they're expensive for what you can buy with them (masks are typically 300, skins I believe are 700). Obviously if there's stuff you REALLY want in the Krystal store that you truly can't wait to unlock, it's your choice, but most of the more desirable gear and skins can easily be unlocked just by playing them game for a few days (Towers of Time, Character Towers and the Krypt), especially nowadays (it was truly gruelling at launch to get what you wanted, now it's pretty easy).

So I'd recommend to save your Krystals for the rare DLC skin bundles that are otherwise unavailable, such as Cangaceiro Kano, Kold War Skarlet, 95 movie Shang Tsung without the jacket, MK2 female ninja skins etc. that come up every few weeks/months on limited offer (just do the character towers first to get those intros as they increase the cost of the bundles immensely unless you've already unlocked them in which case they're deducted from the cost).

At present, the only exclusive thing on offer is the Robocop Announcer, which I bought solely because I had an excess of krystals and have all the other announcers (I've also never changed from Kronika once I unlocked her), even though I'll never use it.

Use them wisely!