The default text interpreter, tex, is interpreting the underscores as subscripts, so you can either:
(1) escape the underscores with backslashes
figure
Ax_Eq = axes();
plot(magic(3));
leg = { ...
'1-Preset1- ==>0 $'; ...
'2-Preset1\_1- ==>0 $'; ...
'3-Preset1\_2- ==>0 $'};
legend(Ax_Eq,leg,'Location','northwest')
or (2) change the interpreter to 'none'
figure
Ax_Eq = axes();
plot(magic(3));
leg = { ...
'1-Preset1- ==>0 $'; ...
'2-Preset1_1- ==>0 $'; ...
'3-Preset1_2- ==>0 $'};
legend(Ax_Eq,leg,'Location','northwest','Interpreter','none')
Option 2 is probably easier in this case.
