Alas, that is all you're going to get...

I have a Maxxum 7D and a 20 year old 4000AF strobe that has served me well
with my film Maxxums.  Unfortunately, the new digital Maxxums and older
strobes do not communicate with each other beyond triggering the flash
(and zooming the 4000AF's flash head with zoom lenses.)  

But all is not lost...  You now have a chance to (re)learn all those old
techniques used in the days before dinosaurs and computerized auto-
everything camera metering systems.  Find a good reference on using Guide
Numbers (ferzample:  http://www.vividlight.com/articles/1214.htm) and
spend some time with the camera, flash and a mathematically inclined brain
cell (or a calculator if your brain cell is as decrepit as mine...)  

I'm not familiar with the 2800AF, but assume it has some manual controls
like output power level?  Even if not, you still have manual aperture
(f/stop) and ISO controls on the 5D - with these and the instant feedback
you get from digital, you can either calculate or just plain brute-force
trial-and-error the proper settings for a situation.  Might not be too
convenient if you are chasing a moving subject, however...

Otherwise, you will need to save up the money for one of the latest whiz-
bang automagical flash units like the Konica Minolta Maxxum Flash 3600HS D
or 5600HS D.  From what I read these will do pretty much everything up to
and including bringing you breakfast in bed.

...And there's always the pop-up flash on the camera itself for short
range work.

Bob ^,,^

You're about two technology steps behind.  Two workarounds:

1) chimping

Set the flash power to 1/2, 1/4 or 1/8.
Take a test shot of the subject.
Look at the monitor image and histogram
Adjust aperture (and/or) flash power to get a proper exposure

2) Metering (requires an incident flash meter)

Put the meter at the subject, dome towards lens
Fire the flash at a nominal power setting
Read the aperture setting, set that on the camera.
Verify with a test shot (chimping) and adjust as required.
(The chimping method (1) is part of this, so you can really see that the
meter is not really a requirement).

Both of the above work for fixed distance for the flash to the subject.

3) Buy a 5600 or 3600HS flash.  There will be many on the used market.

Cheers,
Alan