Ken,
Helen and JT have partly answered this. However, if it's a Sierra DLL it needs to be in the main Sierra program directory.
The error messages sound very like those that Sierra emits when you make an error with the call to the DLL function.
If it is Sierra, in the technical study settings dialog you need to put into the DDL name box
DLLName.FunctionName
note that both are case sensitive and that you need a dot in between