I don't believe a COM+ application itself has a bitness, but rather (as indicated in the document you referenced) the combination of the type of component and the act of adding a component to an application (and whether the application already contains components) determines this.
If you're using the COM+ Application and COM+ Component actions, those actions call the 32-bit COM interface to the
COM+ Administration classes to manipulate applications and components. I suspect that when adding a component with the COM+ Component action, the column in the table of that document which would apply would be "Install component using 32-bit Component Services Explorer" (although MS would have to give the definitive word on that).
If the COM+ Component action isn't giving the desired result, an alternative to the COM+ Component action would be to use the Install .NET Services action and explicitly call the 32- or 64-bit version of comsvcs.exe as needed.