Plain backslashes are not rendered correctly in man pages. They have to be replaced with \e. So this line becomes:

fprintf(stderr, "Event error: %s\en", strerror(event.data));

As above:

printf("Something was written in '%s'\en", argv[1]);

This is not a successfull execution. That said, we usually express that idiom (perror/exit) with err(3).

"dir" in the usage string is somewhat confusing. I do not see why the argument must name a directory.

It looks strange to set event to one shot, and then loop around re-arming the event after each action.

Arguably, if EV_ERROR is set, the even was not fired. Unconditionally print a message about 'written' is not quite correct, isn't it ?

If this is point is not reached, then what is the sense in writing non-trivial code (and really useless if it is reached, since kernel takes care about closing every open descriptor). Note that you do not close fd in premature exit on kevent(2) error. And you do not close kq at all.