The keys into the stash are actually the stash@{n} items on the left. So try:
git stash apply stash@{0}
(note that in some shells you need to quote "stash@{0}", like zsh, fish and powershell).
Since version 2.11, it’s pretty easy, you can use the N stack number instead of using stash@{n}. So now instead of using:
git stash apply "stash@{n}"
You can type:
git stash apply n
To get list of stashes:
git stash list
In fact stash@{0} is a revision in git that you can switch to… but git stash apply ... should figure out how to DTRT to apply it to your current location.