If you try to define relations while in a drawing, Creo will let you, but it's actually creating those relations in the currently active part or assembly. I don't know what version of Creo the original poster is using, but I'm in Creo 4 and I can define relations that work and reference drawing dimensions. This might be because I have the setting
create_drawing_dims_only no
which causes Creo to save drawing dimensions in the part or assembly. Thus they are available for use in relations in the part or assembly.
My personal opinion is that it's best to do all the relations in the part or assembly (not while in drawing) and then use them as needed in the drawing. I.e. create the parameter, say "length" in the part or assembly, calculate its value via part or assembly relations, then use it on the drawing (within note or in dimension text) by typing &length where I need it. I would not use drawing dimensions for relations, because they are "hidden" while in the part or assembly, and some unfortunate person looking into things in the future will probably not have an easy time. I always use the explicit dimensions of the part/assembly to calculate parameters.
