With .NET enumerated values Java Script or JScript must use the numerical define instead of the text define for the enumeration.
For example to set Duplex to single Pass you could call.
// Defines from Header File: Capability - Duplex = 0x1012 or 4114, Duplexes - SinglePass = 0, DoublePass = 2, Data Type - Int16 = 1
Twain.NegotiateCapability( 4114, 1);
Or if you pass in the final value you can call SetCapability directly.
Twain.SetCapability(4114, 1, 1);
Attached is a text document with all of the numerical defines for the public enumerations of ImageMan.NET Twain.