Many phones have a dedicated menu button (either built into the device or sometimes a "soft button" on the screen itself) but some do not, such as the HTC One V. Some will say "Menu", and others may be an icon (such as vertical dots, horizontal lines, squares, etc).
On the HTC One V, press and hold the app switch button (right-hand button at the bottom of the phone).
For sample images, see this page in the WhatsApp FAQ.
If you are still having problems, then you might try a google search for more information specifying your device name such as searching for HTC One V menu button.
It was also reported by a user quite some time ago that a glitch in the prefs file caused his menus to not appear. Although unconfirmed, it is worth a shot to try resetting the app's prefs if nothing else seems to work. To reset your prefs, following the instructions in this post.Tags: menus not working