Appearently my pList got found, even though XCode didn't say "iPhone Settings Schema". My problem was that the values were not set properly. It is important to know that XCode fills in "shortcuts" for your values, but puts more specific values into the source XML. That's why you have to check out the documentation whether you use the correct identifier.
An example:
Adding a group element will be displayed with the value "Group". That does work if XCode recognises your pList as a settings schema.
If not, you have to use the value PSGroupSpecifier as described in the Documentation.
Furthermore it is important to put in all required values, elsewise your childpane will just not get displayed. So if you have any problems with your nested setting doesn't get displayed try following steps to fix it:
Did you name the referenced pList the same as it is specified in your Childpane File specification without adding .plist?
Did you move your referenced pList with the Finder to the Settings bundle?
Did you check if your referenced elements use the exact values as descripted in the documentation (PSGroupSpecifier instead of Group)? This is especially important when XCode doesn't allow you to change the Property List Type...
Are there any required elements which you didn't add to your Element? If yes, it won't get displayed...
Do think you did everything right and it just doesn't get displayed? Delete the app from the device and clean build.