Unfortunately you haven't told us which character creation kit you have purchased.
In general you do all your mixing and matching in Composer mode. So for example you would load a base character and then take that into Composer mode. From there you will find tabs for the Head and the Body which contain all the individual parts you can mix and match.
If you are still unsure, please tell which pack you purchased and I can be more specific.
Peter
Forum Administrator
www.reallusion.com