Add VBA Reference From Another Application Excel To PowerPoint

I have no problem setting up references from one workbook to another. calling workbook references my class variables in called workbook. I want to do the same thing from a powerpoint file (ie reference my class variables from the called workbook). The class variables are not the excel objects but classes that i have defined for my own application. Is this possible? I tried using the tools>references function in VBE that i used for the workbook to workbook references in powerpoint but the excel workbook does not show up in the references list from powerpoint. Any help would be greatly appreciated.

ds1001, I am not sure about 2013, but up till 2010, you cannot add the reference of one application file to the VBA project of another application file, for example you cannot add a macro enabled excel file or even an add-in excel file on to the VBA project of a PowerPoint presentation.