how do you lock and unlock the keypad on a bold?

As long as dial from home screen (Phone>Menu>Options>General Options) is set to no, you can touch the K key to lock it.

You can find the answer here: Stinsonddog's Blackberry Tips and Tricks ‎(Blackberry Tips & Tricks from the Mods‎)

Wirelessly posted

If you turn off the "dial from home screen" feature, then just typing 'k' from the home screen does it.

You can also set it to do that whenever it goes into the holster from the Options/Password screen

Depending on what you use it for, the button on the left side is mappable in the options->screen/keyboard menu.
That way you can map it to keylock, you press that once and it locks it and then you press * and the call button to unlock.

Hold down star on your bold and that will lock it even if you can dial from the home screen.

I cant seem to find the 'dial from home screen'

if i were you i would just hold down star, letter A. Alot easier, because the dial from home screen feature is actually very useful.