My provisioning profile expired so I created a new CER, distribution certificate, and provisioning profile however when I tried to build the app it failed with an error "no signing certificate found for distribution certificate. Do I need to also generate a new p12 certificate and a new appid?

I have an app built for me by a third party. I am not familiar with app development, XCODE, etc but between myself and a friend with a MAC we have been stumbling around XCODE and the Simulator solely for the purpose of getting two screenshots so I can submit the app to the appstore. So far we have failed. We have succeeded it dragging the .app folder provided by the developer, onto a simulator but it opens and then closes immediately - basically it doesn't run. The developer gave me a .ipa file which unpacked contains a Payload folder and Symbols Foler. The Payload has the .app folder which is what we dragged onto the simulator. Can anyone help and tell us what we are doing wrong. BTW - the app runs successfully on iPhones - we used the Testflight facility for that. Many thanks