You can insert a number of special symbols into Mtext. In the Mtext editor select the symbol icon "@" on the formatting toolbar to get the symbols menu.
But what if you want to insert a symbol not in this list? There are a number of characters in most fonts that can be inserted with some good old fashioned arcane syntax. Notice all the \U+xxxx codes on the menu? Those are the Unicode values for the symbol.
Let's say you wanted to use the less than or equal to symbol. It's not available from AutoCAD's list of symbols, but it is defined in the ROMANS text font. It's Unicode value is \U+2264. How does one find this value? Use the Windows Character Map (Start > All Programs > Accessories > System Tools > Character Map).
In Character Map select the True Type equivalent of the AutoCAD font you're using in your drawing. Locate the desired symbol from the font; it's Unicode value is displayed in the lower left corner of the dialog.
In the Mtext Editor type this Unicode value where you wish to insert the symbol. The special symbol will appear immediately.