I bought a merchant's tab but I can't find it in game?

I bought 2 tabs one is merchant another one is currency, but I can only see the currency stash tab.

You're posting in a PoE1 forum, rather than PoE2. Merchant tabs aren't available in PoE1 yet.

If you are in PoE2, the Merchant Tabs are only accessible through Ange--they aren't meant for long-term item storage, only for storing items that you're trying to sell.

To my understanding, you have to finish the Hostile Takeover quest in Act 4 to unlock the ability to manage your shop at Ange--where you can place items into your Merchant tabs to make them available for sale on the Trade Market.

[edit] Based on another thread, the Manage Shop option only seems to be available from your personal Hideout, not from Kingsmarch or from a Guild Hideout.