I accidentally deleted something I need and now there's a "D" next to it, and it's not usable ingame

How do I make it usable again?

Open and load your plugin with the CK, then Data > select your file > Details. From there, you can sort by "Deleted" to easily find the form(s) and hit the delete key which will mark them "Ignored", meaning they'll be removed entirely from your plugin next save. Backup your plugin first though, just in case.