You do not say if these are color or black and white negatives.  You
mention "cartridge".  That is an APS term (as opposed to "cassette" for
35 mm).  Do you have APS film?

If you are dealing with B&W film, then the best next step would be to
intensify the negatives, either with a 10 minute selenium bath or with
bleach/sepia bathing.  The former gives about +1/2 to =2/3
intensification; the latter, about +2/3 to +1 intensification.

If you are dealing with color film, no negative intensification is
possible.  But there is a method for print intensification, involving
repeated bleaching and redevelopment.  This is known as the Anderson
Process.  Do a Google Groups search under rec.photo.darkroom for this
phrase and you will find the process described.  This is a procedure to
be done by a knowledgeable professional lab (not all of them will know
the process) or in a home darkroom.  It will cause more color dye
couplers to be converted with each reprocessing to heighten the color
without darkening the image.

Francis A. Miniter

You should be able to scan the *negatives* (NOT the prints) and get
significantly more detail from them.  One stop is not that much underexposure,
especially for negative film.  The problem with scanning the prints is it's too
late :)

You should also try to work with Curves instead of Levels for this.  Levels
lets you move three points, the white, black and mid-points.  The details you
want to expand are probably between the mid-point and the black point so you
can't really get to them with Levels.  Using Curves lets you move anyone of 256
points so you can open up the shadows much easier.

See if someone will scan them for you in high bit mode (16 bit in Photoshop,
though the scanner will likely be 12 or 14 bits/channel), then you can almost
always extract significant detail from them (at the cost of tonal compression).
I once scanned slides for someone that had been underexposed by three stops
and was able to get pretty good results from them (I was surprised), and
negative film is much more forgiving than slide film, especially just one stop.

Bill

1 stop is well within the latitude of all negative films on the market
today.  If they were only stop incorrectly exposed you should be able
to compensate easily.    In fact, the original prints should be OK...
You need to study the Histograms to find the problem.   I suspect the
issue is there is  no "black" .. the negs are a little then.
Adjusting the toe of the curve should easily fix the problem.  

Take the prints and film back to the shop and have them redo them (for
free of course).  If the clown behind the counter can't realize the
negs are slightly underexposed, then point it out to him/her, cross
your fingers and hope for the best.  Otherwise, take them elsewhere.

On the other hand if the negs are two stops under, all is not lost,
but life will be more difficult. More manual intervention and more
skill will be required.

Jim

I noticed the exposer compensation icon

Now I have a few prints that are rather dark with

that sounds like far more than one stop under.  modern professional films
can give you 2 stops in either direction, 2.5 and even three can be
printable but you will notice lesser print quality.   With sloppy mini lab
printing, you might be 2 stops under or more.

instead of scanning the poor prints, why not scan the negs, or just look at
them and see what's possibly there.  If the dark areas of the print are
totally clear on the neg (except of course for the orange of the blank film)
then there is nothing there.

what you might have is a slightly under subject, but a dark or black
background, the mini lab ran the film on auto, the machine averaged all that
dark backgound and printed it medium.  this is typical for minilabs.

either print the negs again darker so the backgound goes to its proper dark
and see what happens to the subjects, or have the negs scanned which gives
you more options in trying to pull more detail and repair the contrast range
in photoshop.