merging bodys

im going over a 2009 advanced model book i added some tabs to a body now im told to mirror the tabs when i do it gives me a error message( one or more features used in this mirror feature are no longer in the model) i can see them but the do have a  ? at the end of the features in the model tree. what can i do and what is the question mark about.

thanks for any info

SolidworksPlastics And Mold Design