Hi,
Check the below points:
1. DI API needs to be installed on the machine where you are running your windows application, since SAPbobsCOM.DLL (DI API dll) is used for DI API.
2. In your project, go to Solution and right click on Properties option and in the Compile tab, open Advanced Compile Options... and set the Target CPU as x86 and re-build your project.
Hope this helps.
Regards,
Nitin