Sounds like the mod that is called just that..... 'Dino mod'. If it's not that it may be 'Jurassicraft' (not 100% on the spelling of that) Should be easily found with a google.
As for installing it. You need to make sure of a few things:
First being that the mod has been updated to the version of MC you have (so if you are all up to date then you have 1.3.2).
Next, you need to check the mods thread to make sure that it doesn't need any other mods to work (which as of late, quite a few do). Then usually it's is a case of extracting the mod files to the correct place.
All mods have instructions how to do this, some are simple and just require copying a zip file to a 'mods' folder but some need putting into a java executable (the minecraft .bin file).
If you go to Minecraftforums or Planetminecraft there are loads of resources to help you out (sorry, I would link but am at work and they block anything to do with Minecraft)
Good luck!