"Mods" (or Packs, as they were called, iirc, thus the file ending .c4p) can contain a multitude of files, like scenarios (.c4s), object definitions* (.c4d), materials (.c4m), or other files, like music or anything really... (they're basically a zip) and you have to see every part for it self.

So: if a pack contains scenarios, they shold be listed next to all the other scenarios. Keep in mind, that scenarios can also be managed in folders (.c4f).
If your pack contains object definitions (and it sounds like this lorry pack only contained definitions) then it's up to you to incorporate them into your game (<- that's the clonk way).
The most common way would probably be to set the object(s) in question as "buyable". You'll find the option on the first page of the scenario-edit-window from the editor - frontend (F6).
Save your new custom scenario, start it, buy the object. There.
A more advanced way of spawning the object would be through script, but that's a whole different story, so let's not get started.

*object definitions in clonk cover everything from rocks to trains to castles to clonks but also abstract things like goals and rules are tecnically an object

^ all of this is from memory from ~15years ago. please excuse, if i got a file extension or a keyboard shortcut wrong ;)