I need help importing Videos in Encore CS3 (running on a
Mac)

I want to make a DVD with Encore CS3 (running on a Mac).

I've never used Encore before.
I want to import movie files as Assets.
The only files I've had any luck importing are .mov files (exported by Quicktime
Pro).
The movies I want to import are .AVI and .MP4 files.

When I import .AVI files I receive an error message saying;
'The Software that's used to decode the media is not available on this system.
Installing the correct decoders for the files you are working with may help to
correct the problem.'

When I import .MP4 files I don't get any error massages but encore recognizes
each file as an audio file (Type: Audio File).
When I drag the .MP4 files into the timeline and view them in the monitor I
receive sound but not picture.

It seems to me like I'm missing an important codec but I don't have a clue what
to do.

Encore likes to work with DVD-compliant files, like DV-AVI, MPG2 (transcoded to
DVD specs), PCM-WAV files and the like. MOV files are usually handled pretty
well too, as you have seen.

Your AVIs are probably DivX encoded, or similar, and you'll need to install the
DivX CODEC (free for the PC, but have no clue about the MAC). Those are also
heavily compressed files, and when you transcode before burning the DVD, they'll
be re-compressed more. The quality might suffice, but they will not look good.

I do not know file format conversion programs for the MAC, but there are a ton
of freeware, shareware and full-commercial ones available. DigitalMediaConverter
(for PC) is ~US$40 and works well with a lot of different files in different
schemes, both Audio & Video. You still need the proper CODECs on your
machine to decode and then the proper CODECs to encode during the conversion. It
might take some Googling, but you should be able to find both the file
conversion programs and the necessary CODECs on the Web.

On PC, I use a little freeware program, G-Spot, to tell me all the details of
any problem file. It might be available for MAC, but I do not know. It won't
convert, or fix your files, or your system, but will tell you almost anything
you'll need to know, especially the CODEC used and if your system has that one
installed.

Good luck, and sorry that I know nothing of the software available for the
MAC.

Bill is quite right here.
All AVI means is Audio Video Interleave.
It will not tell you the actual codec any nore than MOV will tell you if it is a
P-JPEG, DV, Blackmagic, Animation or H.264 - all completely different, all MOV
files.
AVI has so many flavours it is untrue.