Q: How do I get iMessage back on my mac if I deleted the app?

Hello AlaysiaSum,

Thank you for using Apple Support Communities. 

I see that you are wanting to get iMessage back onto your MacBook. Since iMessage is an app installed with the OS, I suspect that you removed it from the Dock and not entirely from the computer. To that end, this following information provides how to add and/or remove Dock items:

Adding and removing Dock items

If you want to add an application to the Dock, click the Launchpad icon in the Dock. Then, drag an app icon from the Launchpad to the Dock. The icons in the Dock move aside to make room for the new item. If you want to add a file or folder to the Dock, just drag its icon from any Finder window (or the desktop) and drop it on the Dock.

To remove an item from the Dock, drag its icon an inch or more off the Dock and wait a couple seconds. Then release the icon and it disappears in a poof of smoke. 

Removing an item from the Dock doesn't permanently remove it from your computer. If you want that item back in the Dock, locate the app, file, or folder in the Finder or Launchpad, and simply drag it back into the Dock.

From this article:

Mac Basics: The Dock holds your favorite apps, documents, and more

If in fact you did remove the iMessage app from the computer, more information would be needed. For example, how did you accomplish that?

Best regards.