Unlike standard mods that apply globally as soon as the game launches, data packs in Minecraft are loaded per world unless a modpack like Prominence II manages them automatically.
Adding to a New World
1. From the singleplayer menu, click Create New World.
2. Navigate to the More tab and click the Data Packs button.
3. Click Open Pack Folder and place the Simply Swords All the Smites Data Pack file inside the folder that opens.
4. Return to Minecraft, move the pack from the Available column to the Selected column, and click Done before creating your world.
Adding to an Existing World
1. Select your world in the singleplayer list and click Edit at the bottom of the screen.
2. Click Open World Folder, then open the datapacks subfolder inside it.
3. Drop the data pack file into that datapacks folder.
4. Load into your world and equip a supported greatsword, greathammer, or Watcher weapon in your main hand to confirm the Divine Smite spell is now attached to the item.