I am having trouble adding traincraft as resource pack in Minecraft on Mac

Mods are not resourcepacks. You need Forge for 1.7.10, and a mods folder.

oh, how do you do that?

There's a number of ways, default is just installing forge itself, then putting the traincraft jar file in the "mods" folder of the install:
https://files.minecraftforge.net/net/minecraftforge/forge/index_1.7.10.html

there's also modpack loaders which give you pre-setup installs for entire mod sets, typically we recommend technic:
https://www.technicpack.net/modpacks?q=traincraft

There's also curseforge, although it's not fully supported because their client is known to be unreliable.
https://www.curseforge.com/download/app