Sorry, I don't know what you are referring to when you say "XPression CG". You need to be clear about whether they are using SDI video, or the RossLinq interface. XPression Prime using SDI video will be able to use a clock. XPression Designer with RossLinq will only be stills.
If you need additional help I suggest calling the Ross Video Tech Support line and they can talk you through this over the phone.
#XPression