One method would be:
find ./ -name "*.m" -print0 | xargs -0 genstrings -o en.lproj
xargs is a nice chunk of shell-foo. It will take strings on standard in and convert them into arguments for the next function. This will populate your genstrings command with every .m file beneath the current directory.
This answer handels spaces in the used path so it is more robust. You should use it to avoid skipping files when processing your source files.
Edit: as said in the comments and in other answers, *.m should be quoted.