For a pulldown menu, I believe that is controlled by your operating system.
For a static text control, or a text object in the overlay, you can set the 'FontWeight' Property to 'Bold'.
text(yourString, 'FontWeight', 'Bold');
From the help for "Text Properties"
FontWeight
{normal} | bold | light | demi
Weight of text characters. MATLAB uses this property to select a font from those available on your particular system. Generally, setting this property to bold or demi causes MATLAB to use a bold font.
