Micros E7 error "Interophelper"

[o1raul]

Hello,
I hope someone can help me. I have a micros e7 ws4xl currently running, I purchased an additional terminal (WS4)from ebay in working order hoping that I could simply update from pc and have running until I could find a key. fast forward 7 months. I have been unable to get the workstation to run the e7 software.
I have upgraded the e7 software and cal software but this has not worked and almost cost me the ws4xl till.
The only difference that I think is causing problem is that the ws4 is running windows CE4 and the ws4xl is running CE6.

When I download the from the PC all looks ok but when it try's to load e7 I get the following error
"Error
An Unexpected error has occurred in e7.exe.
Select Quit and then restart this program, or select Details for more information.
Cant find PInvoke DLL
'InteropHelper.dll'."

When I select Details I get the following
"Error
e7.exe
MissingMethodException
Can't find PInvoke DLL 'InteropHelper.dll'. at MicrosSystems.E7.POSMain.MainForm.Main()"

Please can anyone help or did I buy a dud?