Is Splatterladder HD Pack free?

Yes. Splatterladder HD Pack is released under Free fan content (see the ModDB page) and costs nothing. Neither it nor Wolfenstein: Enemy Territory has any paid content.

What do I need to run Splatterladder HD Pack?

Wolfenstein: Enemy Territory, which is itself free. Splatterladder HD Pack is an add-on and will not run standalone.

How do I install Splatterladder HD Pack?

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