I didn't explain the problem you had:
When you evaluated the snippet of code, it was compiled to a temporary #DoIt method (not installed into the method dictionary). Then, the example serialized a blockclosure, that was defined into the not-installed #DoIt. By default, Fuel serializes the CompiledMethods just as a class name and a selector. So, when you try to materialize the blockclosure, Fuel asks the method dictionary for the #DoIt, which is not there. So signals an error. Using FuelMetalevel, you can tell Fuel to fully serialize the CompiledMethods, so DoIt can be serialized and materialized, no matter if it is installed or not.
Just to complete the idea... if the block closure were installed in a class, then there will be no error. If you file-in the attached file and evaluate:
FLSerializer example.
I works, so you can see a string printed in the Transcript.
Martín