For most devices the menu button is a physical button on your phone. It is not part of the screen. The icon for the menu button will look different on different phones. Here are a few examples of what the menu button could look like:

If you have Android 4.0 (also known as Ice Cream Sandwich), your menu button will be on screen and will look like this:

For some Android 4.2 phones, the menu button will be in the action bar:

For phones without a menu button:
To use the menu button on your phone, you must long press on the app switch button:

Cheers,
WhatsApp Support Team