Zoom in and out without a mouse wheel I am using blender on a tablet, for those times when I can't plug a mouse into the tablet, I would like to still be able to zoom in and out on the viewport. 

What ways are there to zoom in and out other than using the mouse wheel? I can't find any buttons or input boxes in the user interface that control the zoom of the view port.

Is it possible to enter a value somewhere for how far in or out the viewport is zoomed?
The simplest way to zoom with a tablet, in my opinion, is to use Emulate 3 Button Mouse from File > User Preferences > Input.

This allows you to zoom in and out with Alt+Ctrl+LMB  drag.

Then you can also orbit with Alt+LMB  drag. And you can pan with Shift+Alt+LMB  drag.