To get the whistle and more bombs in level 5, from your starting point (the first room of the level) go:

Up 1 screen
Bomb left 1 screen; go through
Bomb left 1 screen; go through
Take the passage way; from where you appear go:
Down 1 screen
Bomb right 1 screen; go through and get your bombs for 100 rupees.
Go back left 1 screen
Back up one screen (you should be in the secret passage room, now)
Left 1 screen; push the middle block to the underground passage with the whistle.

Here's a map to help you through, if you're interested:

http://db.gamefaqs.com/console/nes/..._dungeon_05.gif

Good luck!