You may need to manually register imsigx5.dll and gxext15.dll using regsvr32.
If that doesn't help, you may need to re-import the TurboCAD 15 versions of the IMSIGX15.dll and GXEXT15.dll type libraries, and re-compile using those versions of IMSIGX_Tlb and GXEXT_Tlb in your project.
Finally, does your version of TurboCAD include the solid boolean functions? These may not work in all TurboCAD packages. I have only tested this in TurboCAD Pro.