It still sounds like you’re using the bottom menu right now. Does it look like this?
If you click here, it’ll hide the bottom menu on your screen:
The bottom menu is only for placing or selling items really, so you won’t be using it at all to equip things like jetpacks. You’ll be using only the top menu for that.
Jetpacks are considered a wearable, so first you need to go to the top of your screen and click this button to go to the Wearable Hot Bar.
After that you’ll find a button here for your jetpacks:
Now you’ll drag the jetpack into one of the wearable hotbar slots to use it.
Hope that helped!