I want to label a figure in MATLAB with "smaller equal" symbol, (for example, my variable runs from 100 to 1000, so I want to label like: 100 =< x =< 1000).
I did not understand correctly que question, I assumed you was trying to do that at the command line. Concerning to the issue, I believe that text caracters are allowed to be inserted at the text box.
Anyway, what if you try just copy-paste the following symbol to there ?