There is no space in between the -D and the __NO_FLOAT__, so the option is
"-D__NO_FLOAT__" and not "-D __NO_FLOAT__ "
The -D option is in the GUI in the language panel and is called Preprocessor definitions.
Just enable it and write __NO_FLOAT__ into the field and the compiler will add -D__NO_FLOAT__ for you.
Note that the main effect of -D__NO_FLOAT__ is when compiling the library, more specifically printf.c.
There is already a pre built library with -D__NO_FLOAT__ , see lib\hc12c\readme.txt, so unless you compile printf.c on your own, the effect of -D__NO_FLOAT__ is not that significant. If you do not compile printf.c and you do use (s)printf, be sure to link against the right library.
Daniel