Okay, so I wasn't sure what you meant by replacing the default Grunt model - either you want to have all of your Grunts across all WC3 maps locally replaced with the SoC Grunt, or you're trying to use the SoC Grunt as a model in your map.
If you're looking to use the model in your map, then simply import WC2Grunt.mdx, WC2Grunt_Portrait.mdx and WC2Grunt.blp, and change the path of WC2Grunt.blp to remove the "war3mapimported\" prefix, meaning the path has to be simply "WC2Grunt.blp".
If you're looking to replace the default Grunt across all games locally (only you can see this change), then it takes a little bit more effort. You'll need to create folders inside folders in your main Warcraft 3 installation folder. This can be done through the following steps:
You need to place the .MDX model in your WC3 folder, under the path:
"Units\Orc\Grunt\grunt.mdx"
The portrait, correspondingly, goes under:
"Units\Orc\Grunt\grunt_portrait.mdx"
The texture (.blp file) needs to be in the same folder as the model files, and it has to be called: "WC2Grunt.blp".
The icons, if you want to use them, go under the folder:
"ReplaceableTextures\CommandButtons\BTNGrunt.blp"
"ReplaceableTextures\CommandButtonsDisabled\DISBTNGrunt.blp"