You can enter a matrix (there's no such word as matrice) using LaTeX like this:
[ tex ]\ begin{bmatrix}1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \ end{bmatrix}[ /tex ]
This LaTeX code (with the extra spaces removed from the tex tags and before begin and end) produces this matrix:
[tex]\begin{bmatrix}1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{bmatrix}[/tex]
There are several other ways to do this, of which this technique is probably the simplest. If you want the matrix surround with parentheses instead of brackets, use pmatrix in both places.