I'm a runner. My music and playlists are on Google Music. I have all of my music downloaded to my device. I can't seem to figure out how to get apps like Runtastic or Nike+ to locate my Google Play Music. Any pointers? Do I need to manually move the files around on my device (android...Samsung GS3)?
Short answer you cant. Google Music provides no API to access you music and playlists that are stored in the Google Music Cloud.
You can dump music from a playlist in the Samsung->Music folder and then choose to shuffle the library (or just play). This will work similarly to a playlist from itunes/Google Music. Use Explorer to simply copy/paste the files into the folder on your phone.
I've had the same troubles. I had an iPhone for years and didn't even consider for a moment that accessing the music through your running app would even be remotely complicated as with iOS all all just works.
When I switched to Android I found that it's actually a really difficult ask. In fact the running app i have used for years, Map My Run doesn't even have a music option so I've begrudgingly gone back to Nike + (after trying several others in the process) as this allows me to access the music on my phone whilst retaining a half decent running app in the process (something that some of the others didn't quite manage to deliver).
I now use an app called Media Monkey, which is a bit like iTunes, although closer to Winamp, to move the music I want on my phone across to the device. It's annoying as I spent hours uploading all my music to Google Play and setting up my playlists only to discover that I can't access it through the Nike + app. I pretty much only ever listen to the music on my phone when I run so the day Google Play and Nike start talking to each other will be a very happy day indeed! :)