The member was likely removed by the linker, and thus doesn't exist in the assembly at runtime.  There are several solutions to this:

• Add the [Preserve] attribute to the member.  This will prevent the linker from removing it.
• When invoking mtouch, use the -nolink or -linksdkonly options.
  • -nolink disables all linking.
  • -linksdkonly will only link MonoTouch-provided assemblies, such as monotouch.dll.

Note that assemblies are linked so that the resulting executable is smaller; thus, disabling linking may result in a larger executable than is desirable.