Where's the universe frontier or the universe is within...?

Depends on who you ask becuase there are many diffrent theroys.
But most likely there is no "edge" and even if there was there would
be no way to get to it because it would be moveing away faster then
you could ever go.  With universal expensoion any edge would be
moveing away from you at the speed of light and would be similer to
the effect of someone who had fallen into a black hole's event
horizen.
Your other part is the universe within?  Is kinda vague but I'll
answer it as I know it.  The universe could be within something, but
it would be something that would be indescriable within our know laws
of phyiscs.
What is beyond the universe is similar to the question what happened
"before" the big bang, again the answer is nothing that we can
describe becuase everything that we know of is within the universe.

The short answer to your question is that there isn't considered to be
any "edge" of the Universe in the formal sense, and that the Universe
isn't "within" anything, but rather contains "every thing."

The best information we have here is that if you could "look" toward
where  the end of the universe might be, you would only see further
and further back in time.  This is because light travels at a defined
rate, and so can only give us something to see if we can see it.  This
means that conceivably, you could see back to the beginning of the
Universe if you could see that far, but as yet, we are unable to
accomplish this.  The movie "Space Odyssey 2001" addressed issues
along these same lines.

This all has to do with Einstien's theory of a curved space-time
continuum and is very difficult to articulate.  You might take a look
at this website:  http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961202c.html
 And also  perform a search on Google with terms like these: 
"universe edge end time"  And see if you can find more interesting
information.  A book that you might want to look at is modern
Cosmologist Steven Hawking's classic "A Brief History of Time, " or
his more accessible "The Universe in a Nutshell."

there are three main theories that I can think of.

1. the universe is limited and within "SOMETHING"

2. the universe is limited but as you travel in one direction
continuously you eventually end up where you started, but there is no
defined beginning, end, centre or edge.

3. the universe is endless and unlimited. This is hard for the human
mind to comprehend as we prefer to think of everything as having a
begining, an end and an existence to be something.  Perhaps we must
ask why the universe must have an end before we ask where it is and in
what way it exists.

If there is an end, does that mean the the idea of "nothing" cannot
exist? Is every bit of what we know and understand filled with some
form of energy?

Perhaps that is the end of the universe. Where energy in any form does
not exist and will never exist, and beyond is something else. Maybe we
can say that the edge of the universe is the furthest that the
smallest possible, portion of energy can possibly travel from all
other energy.