Here is a guide to how to use the correct snapshot version
Press the 'Edit Profile' button on the Minecraft launcher, this is the screen before the main game is loaded.
Then set the following options within this screen:
Select the correct snapshot for the map:
- Prismatic -> snapshot 15w42a
- Workshop Wars: Christmas Edition -> snapshot 15w47c
Then press 'Save Profile'
Note: if the resource pack shows an incompatibility error you can ignore this as it actually works fine on this snapshot.
Note: if the snapshot version is not showing then please do the following:
1) Download the jar and the json file here: http://minecraft.gamepedia.com/15w42a
2) Create a folder called 15w42a in your .minecraft/versions folder
3) Place the 2 files in the new folder
4) Reload game and the version should show up in the launcher
Hope that helps!