I am assuming you want to highlight text in editor and not the code. Text can be highlighted in Live Editor (.mlx) files using bold, italics and other formatting options. These can be customised using Preference Panel --> MATLAB --> Editor/Debugger --> Autoformatting. Standard keyboard shortcuts (Ctrl+B, Ctrl+I, etc.) can also be used for the same.
Ctrl+F (Find feature) can be used to search for specific text, and MATLAB will highlight all occurrences of that text.
I hope this answers your query!