After building the .ipa, yes, you need to sign it. You're on the right track with the XReSign app.
I don't remember the steps exactly, but:
The mobile provisioning file can be downloaded from the Apple Developer website. To do this you need to login with your Apple ID (it will ask you to create an Apple Developer account if you don't have one). From your account page, there should be a Certificates and Profiles page, and from there you can generate and download it. (Note: You can only test with this profile as to publish your app you need to enroll in the Developer Program)
Use XReSign to sign the .ipa using the downloaded profile.
Hope it works!