Why can't I see Mars clearly? I just bought my first telescope - a Celestron CDC 1100.  It came with a 40mm lens so with a primary focal length of 2800mm the power should be 70x.

My problem is that I've looked at Sirius and Mars and they look clearer to my naked eye than with my telescope.

I am able to focus from way out so I can see the 'blind spot' of the secondary mirror, go through 'focused' and then continue moving the mirror in the same direction until I again see the blind spot.  So I know I'm focusing through when it should be 'good'.  But, like I said, what I see is a relatively unfocused spot of light that is clearer and steadier with my naked eye.

Admittedly, my 'seeing conditions' aren't very good since I am looking through my window in a suburban neighborhood where light pollution is pretty bad.  That being said, I still see Mars and Sirius quite brightly - it's just that they never come into clear focus - nor are they any larger than when I look using my eyes.  And when I move my eye around the eyepiece I see streaks of light in different directions originating from the target.

There are many stars to be seen, though they are just pinpricks as I would expect given the seeing conditions.  What I mean to say is that I cannot see all those stars with my naked eye but can see tens or hundreds more looking through the scope so I know it is gathering plenty of light as an 11" scope should.

I tried both with and without the diagonal with no discernable difference.

I have not collimated but supposedly it was collimated before leaving the factory and I would hope that even if it were off a bit from shipping I would still be able to see a sizable though fuzzy image of Mars instead of just a small speck of orangish light.

Any thoughts as to what I may have done wrong or what may be causing this lack of clarity?
You're experiencing a few issues all of which have one solution. Take the Scope outside. Let it acclimate to the ambient air tempurature, and then try looking.

The 1st issue is the mirror needs to be the same temp as the air you're observing in. Since you are indoors you have a very warm optical path this causes distortion and trouble focusing.

2: Because you're looking through a piece of window glass (or more if it's a multipane window) you're introducing distortions that way. It's no wonder you can't reach focus.

Light pollution won't really affect Mars or the other planets too much, so that's not your worry.

Why are you observing from inside anyway?