nadiya, please don't post on outdated threads like this one, make a new one

To answer your question though, you need to open the .mat file with Notepad.
Something like this should show:
material
rockMat{
technique
{
pass
{
ambient 0.1 0.1 0.1
cull_hardware none
cull_software none
texture_unit
{
texture rockTex.jpg
}
}
}
}
That is the part you type in, not the name of the .material file.