Did you really try searching for this yourself?

I just typed your exact question into Google, and the very first result told me that Blender can not export to that format, but suggested simply exporting to one of the many other formats that MilkShape 3d can import and proceeding from there; if you just needed to get the Blender file into MilkShape 3d job done and you can move on. If you really need the ascii format for some reason you can export again from MilkShape into that format.

The second result was a Blender exporter for MilkShape 3d MS3D format, which will also solve your problem if you just need to get the file from Blender into MilkShape 3d, or which could be your intermediate step if you really need that ASCII format for some reason.

We're happy to help people, but honestly based on this and your other recent question it doesn't really look like you're even trying to find your own answers first. You're also not explaining what you're trying to do in much detail -- if you tell us why you think you need to export to that particular format we could recommend alternatives.

Hope that helps! :)