the most important word is "Ad Hoc"
maybe the best is just to follow the list in the developer.apple.com -> program portal -> Distribution tab (in the link of team, certificates, devices, app id's , provisioning)
the problem is that I at least tend to search the web for links about this subject before looking in the apple dev portal....
in distribution tab open up the prepare tab and read the steps...
problem with me was the key - distribution profile combination
in xcode in projectpreferences the distribution profile was indicated but also the error of no combination of key and profile found
in the keychain i had to select a private key and drag the profile into it, than the key was combined with the profile like on the image:

other not apple link info:
intro:
http://www.appletell.com/apple/comment/app-store-development-part-2-provisioning-and-ad-hoc/
http://www.appletell.com/apple/comment/iphone-developer-intervention-week-7-beta-testing/
how to make an ad hoc provisioning profile:
http://www.manyfriends.com/wiki/iphone/index.php/Create_an_ad-hoc_provisioning_profile#Create_the_ad-hoc_profile
a list of steps to follow after that
http://furbo.org/2008/08/06/beta-testing-on-iphone-20/
a free app to use to get betatester to send you the UDID
http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=285691333&mt=8
there is a site for global beta testing (not tested by us yet:-)
http://ibetatest.com/iphone/controllers/welcome/