The method that most of the above posts suggest make the assumption that both your cave map and your topo map are aligned to the exact same North reference (hopefully Nt, "true North" as opposed to "magnetic North"). They also leave it up to you to align the cave with its correct position. The following suggestions may not qualify as the "easiest way" necessarily, but they'll scale better than simply resizing your map in Photoshop (which really is just a simple math problem).
A more modern approach, assuming you have the survey data available, is to export a Google Earth KML file or an ESRI Shapefile from
Walls,
Compass, or other cave survey software (Therion, Survex, ...). The output lineplot (or "volume plan" based on LRUDs) will be georeferenced so that it appears in the correct place geographically, oriented correctly, and (optionally) with correct elevation data for passages. But teaching you to do this is beyond the scope of this forum thread! There's a moderate learning curve if you're not already managing your data in one of these programs, and even then, it can be tricky to properly export a Shapefile given issues with datums, units, and control points.
An intermediate solution would be to take your scanned map and "georeference" it with GIS software, assuming you have at least two known locations on your map (like two entrances).
Quantum GIS is a free and easy-to-use GIS suite. Once you've georeferenced the image, you should be able to import it into Google Earth, and probably other consumer GIS type applications like Terrain Navigator, as well as any other GIS software. I don't know anything about Topo USA 9. Here's an example to show how to georeference any scanned image using QGIS:
http://qgis.spatialthoughts.com/2012/02 ... heets.html You could probably hack around this method using the scalebar of your scanned map as an additional known point, but ideally you'd want to use several points spread throughout the map if possible.