Are you using CD to get to TurboCAD's Programs folder before running tlibimp? That's the way I used it; it put the resulting IMSIGX_TLB.pas and IMSIGX_TLB.dcu files in the same directory as the dll.
As for syntax errors: yes, I had some similar problems. Most of them are due to tlibimp not recognizing some of the properties as readonly -- get rid of the write portion of the properties that throw errors.
I also had to rename the Selection interface to IMSISelection -- looks like a naming conflict with something already in Delphi. This involved changing a number of lines -- everywhere the unit expects a Selection interface, change it to IMSISelection.
Finally, the TAppliation.DisConnect procedure is wrong somehow; for now I have simply commented out everything between the begin and end lines for that procedure.
Have a look at my modified IMSIGX_TLB.pas file in the Delphi sample project I just posted in the User Contributions area.