Okay so I have the old .dll but it wants a serial number. Hmmmm.  Tried Command Center but does not show up there.  Any ideas how I can activate this plug in so I can use it for a legacy song of mine?

Only Z3ta+2 has a separate installer written for Command Center. The original Z3ta+ has been bundled for years with SONAR Studio/Producer/Professional and Platinum. When installed from one of those versions of SONAR no extra activation is required. However, when installing from the separately purchased Z3ta+ installer the serial number from your user account is required.
 
FWIW, Z3ta+ is in the Platinum Instrument Collection installer. I am not sure what installers come with SONAR Professional. Both of these should be in the Command Center. For X3 and older use the advanced install option and select the synth from the custom pick list.
 
 

Thanks, I figured it out ;-)