Did somebody say Displace? Hehehe.
Since he points to the folder but not a specific file, I would assume that he means to try any and all of them.
Time to step on Mark's toes. I took a quit peek at the stock D-Maps. Personally, I don't think they fit well with the tutorial. Either too sharp or too contrived (contrived in a manner best left for other things).
Blech! Let's make our own. A Stroker variation on a golden oldie.
Start a new document, around 300x300.
Hit 'd' to get the default black and white colours.
Go to the Channels palette.
In the Red channel, run Clouds/Difference Clouds to taste.
In the Green channel, run Clouds/Difference Clouds to taste.
Blue channel is up in the air. I like to fill mine with 50% gray.
Go back to the composite RGB channel.
You should now have a bunch of pretty colours. Ohh... ahh...
From here, you have a lot of options. You can run a myriad of filters to jazz it up. Feel free to experiment.
For the BG FX tutorial, I recommend some Gaussian Blur. This will give the D-Map a smooth, wavy, watery kind of feel to the distortion. I think this fits well with the tutorial.
Save it as a PSD and use it as a D-Map when you run Displace.
That's how I would go about it. 3[
Now, way back when, I did go through the BG FX tutorial. It's one of the few tutorials that I actually did. When I went through it all those years ago, I left out the Displace step. Came out fine for me.
Do you toes hurt, Mark?