How do you get HM7 in Pokemon Mystery Dungeon red rescue team?

To obtain the HM Fly in Pokemon Mystery Dungeon, you must first have the Friend Area "Dragon Cave". This is easily done by entering the following Wondermail Code and completing the mission:

H?08 ?9S? ?Q64

?P?! 84S0 +F?!

This will unlock Wyvern Hill. Make your way through the dungeon to the 29th or 30th floor, where fly is located. You will need a Key to access it through the keyhole.

If you lose the HM, you can always come back to get it again. However, you are only allowed to have one HM FLy at all times but, if you go to the same place again while you still have fly it will be a link cable instead which helps to evolve certain Pokemon

The HM Fly is used to access through Sky Tower again after the first time. which will also let you have a chance of recruiting rayquaza

but where do you get the key?

u get the key in Solar Cave...if i remember right....