can't access in-app purchase on the app store

No, the inApp purchase just says, hey this app has in app purchases that you can get if you want to. You need to find the get button or buy or download button and click on that then enter your credentials.
HTH. also, welcome to applevis!!!!!

To be a bit ore clear:

1. Launch the app for which it has an in app purchase
2. Go to settings, or where ever it lists prices for the purchases
3. Tap the price or buy or what ever the instructions say.
4. Enter in your password.
5. Hit done in the lower right

Sometimes the in app purchases will be in the settings, some times in the main body of the app. It's best to go exploring. I've been an iPhone user for 5 years so this comes from a lot of experience and helping people through this.

Good luck.

Pretty much the first commenter was right. parents were mad their kids were purchasing stuff they had no idea apps could charge them, IE. Junior was racking up Mommy's crdit card bills like whoa. So now Apple has to legally say, hey this free game is free, unless you want to unlock soemthing, say levels, or characters, also some features of apps are covered under this category. at the moment, I'm not thiking of any app that i can name, oh except like blind fold wild card. that works fine, free, but if you want to add on the ability to I think use multiple decks etc, then you spned the four bucks, american curency to do that. hopei was a bit more clear. if not, it's late. :)

Usually, In App purchases are made in the app, not in the app store. Have a look in the app's settings section, or perhaps something like a "Shop" or options section.

There is usually a unlock or buy button somewhere in the game's settings screen. That's how you unlock other modes of play like campaign mode. In the game Solara, for instance, theres a button labeled app store and treasure that will take you to a shop where you can support the devs and do other stuff with gold and money.
HTH.