"Charge" is not a button. When you see a reference to charge, it means that you need to hold the specified command for two seconds.
For example, let's say that you are playing with Blanka and want to perform his Rolling Thunder Attack. You would need to do this:
The charge means that you need to hold left for two seconds, then quickly tap right and the punch button.
As for the HML, it means Heavy + Medium + Light (Punch or Kick) at the same time. You don't need to press the three buttons together though, there is a button that does the HML function (L1 for HML punch and L2 for HML kick).
You can also reassign those buttons if you feel uncomfortable with the L1 and L2 buttons.