Is StarCraft: Mass Recall free?

Yes. StarCraft: Mass Recall is released under Freeware / Community and costs nothing. Neither it nor StarCraft has any paid content.

What do I need to run StarCraft: Mass Recall?

StarCraft, which is itself free. StarCraft: Mass Recall is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from StarCraft's mods list. You can also install it by hand — full steps are on this page.

Roughly 1.4 GB, on top of the base StarCraft installation.

StarCraft: Mass Recall — Free StarCraft Mod · PlayBound