As it tends to be with different ports (or "ports" in Blood's case) of a game, loading up mods can be a little tricky to keep track of over multiple ports. As far as I saw there was no explanation on how to install them in Fresh Supply, but after tinkering around I've got it worked out.

If you don't know where your install folders are by default, on Windows it's usually along the lines of C:\Program Files\Steam\steamapps\common\Blood or C:\Program Files (x86)\Steam\steamapps\common\Blood, if you have a custom install folder I assume you already know where it is. I don't know where it installs to on Mac or Linux yet, so if someone has the info on that, let me know so I can add it in here.

Have the map pack you want downloaded and ready to unzip - or already unzipped and in another folder if you want.

• Navigate to your Blood folder, and then to your "addons" folder inside it. Cryptic Passage should be preinstalled.
  
• Create a new folder, you can probably name it whatever but for the sake of good housekeeping name it with the name of the map pack you're installing. In my case, Death Wish.
  
• Open that folder and dump the contents of your downloaded map pack inside. It should look a little something like this:

And that's all the "work" done. Boot up Fresh Supply and head to the in-game options, then Manage Addons, and you should see the new map pack next to Cryptic Passage. All you need to do after that is click on it, load the addon, and then start a new game. As long as you're in the habit of saving your game, you won't even need to navigate the menus when you boot the game again - you can load your savefile just fine.

One final note: if the map pack you're trying to install doesn't have a .ini file, Fresh Supply won't recognize it. It also won't recognize it if there are multiple .ini files in the same addon folder.

Have fun!