Can not see anything thru the lens

I answered your question already-- TURN THE SCOPE around you have it mounted backwards! The eyepiece end is pointed UP. Like the picture above in your question.

All astronomical telescopes have upside down images. There is no up or down in space; and it requires more glass elements to turn the image upright. More glass means less light will reach your eye.

Here is the dealer for Busnell call them:
http://www.opticsplanet.net/telescope-faq.html

Practice focusing during the day on a distant object at least 100 yards away-- use the eyepiece with the LARGEST number written on it-- and DO NOT use any 2x or 3x barlow to multiple the power.

The moon should be your first target at night.

Put the diagonal into the telescope-- then put the eyepiece that has the LARGEST number written on it. This is the lowest magnification. DO NOT USE THE BARLOW 2x multiplier !!

Take the scope outside during the day time and practice focusing on a distant object. The moon should be the first night time target you try to see.

Try pointing it at the moon first-- with the 20mm and practice focusing it. The moon is up just after dark right now.

Put the lowest power eyepiece in the telescope-- that's the one with the LARGEST number on it.

Go outside and practice focusing on a distant object at least 100 yards away-- turn the focus knob slowly.

Just got a 78-8831 and set it up according to instructions. After inserting lens into eyepiece, I can't see anything at all. Dont worry, I've removed the front cap from the the main telescope.

Something leads to believe I'm either missing something real fundamental or my telescope is broken. Please help

Not worth repairing-- a new focuser for your telescope would cost as much or more than the telescope-- unless you can find a discarded telescope of the same model..... for parts.

There are a bunch of Sky Tour telescopes ---

http://www.opticsplanet.net/bushnell-warranty.html

which model you actually have??

I am assuming you have what is called a Dobsonian mounted reflector like this one:

http://www.bushnell.com/manuals/telescopes/78-4500_Voyager.pdf

Is that correct?