how do I execute a void-returning procedure in pgsql 9.1?
There is no problem with how you call your function.  The error you get comes from executing it if and only if cnt is greater than zero.  The error you get is

ERROR:  query has no destination for result data
HINT:  If you want to discard the results of a SELECT, use PERFORM instead.
CONTEXT:  PL/pgSQL function "waitris" line 9 at SQL statement


and it clearly states that the problem lies in the SELECT pg_sleep(1); line.  That is the place you should replace SELECT with PERFORM.

Otherwise your loop is futile without a CONTINUE.  And even if you add that, behaviour of the loop is hard to follow.  You'd better use a WHILE loop instead.  And still one more: as you see, I changed the variable name count to cnt for better readability.