You can find the multitexture terrain shader in the templates folder of your main Gamestudio folder (update to 6.40+ first). Once you've started a new project with a function main, copy the terrain shader into your new project folder and write include "mtlmultiterrain.wdl" ( or whatever its called-I cant remember now) at the top of your script. Lastly load your unskinned terrain into WED and rightclick-properties-behaviour etc- the mtlterrain action should be in the actions list now. Assign it to your terrain, build as an entity and save.
Once youve done that you can start experimenting with terrain skins- make a base skin of 1024 to stretch over the whole terrain + a detailmap(s) of 512 and add them to your terrain via the skin editor in MED. Now go back to WED, rightclick your terrain again and modify the skin settings- you want 1 for your base texture and 20+ fo your detail(s). As far as the other stuff is concerned you'll have to play around with settings and/or run a search on the forum if u havent already done so. Also check the manual for other info about this feature of the new release. Good luck.