How do I use a Lure Module?
- Go to a pokestop.
- Activate the pokestop.
- Touch the white space above the spinning circle (the shape of the lure module).
- Confirm to use the lure module.
- Wait around for half an hour and catch lots of pokemon.
Happy huntung!
If you need a source go to https://www.reddit.com/r/pokemongo/comments/4rst52/cant_use_lure_module/, or trust experience.
After spinning the circle a white blank spot should appear above the pokéstop, tap that to install your lure.
Source