Saving a property list in a file wrapper takes two steps. First, call NSPropertyListSerialization’s dataFromPropertyList: or dataWithPropertyList: methods to create a property list. The dataWithPropertyList: method was introduced in Mac OS X 10.6. Use dataFromPropertyList: if you are supporting earlier versions of Mac OS X.

NSMutableDictionary* root;

NSError* error;

NSData* propertyList = [NSPropertyListSerialization dataWithPropertyList:root format:NSPropertyListXMLFormat_v1_0 options:0 error:&error];

Second, pass the property list you created as the first argument to NSFileWrapper's addRegularFileWithContents: method.

NSFileWrapper* wrapper;

[wrapper addRegularFileWithContents:propertyList preferredFilename:@"MyFile.plist"];

My Working with Cocoa File Packages post has detailed information on file packages. Read Apple's Property List Programming Guide for more information on property lists.