My first foray into Apps for Mobile Computers.
I've a MC2100 and just downloaded the Symbol Mobility Dev Kit for .Net v1.7, apparently the latest kit. When I try to run code samples (that came with the Dev Kit),as soon as I hit any kind of Scanner related code I get the "UnmarshalStructInfoFrom" error. Someone did report a similar issue on this site and later said that updating the Symbol and Symbol.Barcode dlls fixed the issue, not sure if this meant getting a new version of the dlls (if so which one?) or simply delete and re-add references which I did, but to no avail.
You are using quite an old version of the development kit. The latest version is 2.6 and has been rebranded EMDK. Since the MC2100 was just released there is no support for it in earlier EMDKs.