So you're trying to load a CTB/PC3 before a plot? If thats the case why not use a quickplot kind of function, a button/command that has all plotting parameters in it, such as:
ThisDrawing.SendCommand "-plot" & vbCr & "y" & vbCr & "Model" & vbCr
& "Oce TDS600 3.x.pc3" & vbCr & "ARCH expand D (36.00 x 24.00 Inches)" & vbCr
& "inches" & vbCr & "Landscape" & vbCr & "no" & vbCr & "extents" & vbCr & "fit"
& vbCr & "Center" & vbCr & "yes" & vbCr & "comed36x24.ctb" & vbCr & "yes" & vbCr
& "n" & vbCr & vbCr & vbCr & vbCr
This of course doesn't really answer your original question, but it does get the job done (if thats what you need to do).