writing NSDictionary to plist in my app bundle

Johan is right -- you're not allowed to modify your app's bundle. You have two good options for where to save your dictionary: the documents directory, which is backed up by iTunes when a user syncs their device; or the caches directory, which is not backed up. If you don't need to have the data backed up, put it in caches so you don't slow down syncing. You can get the directory paths like so:

NSString *documentsDirectoryPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

NSString *cachesDirectoryPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0];

It doesn't get saved because you can't write into your app's bundle. Save your file elsewhere, in the documents folder for example.