Pixelmator dither ??
 

In Photoshop I can export to the web and add 'dither'.

In Pixelmator, because I can't do this, images (PNG 8 bit) can look dreadful.

What do I do?

No idea what Pixelmator is - however if you're having filesize issues, which I guess may be why you're looking at resolving dither, due to large save files, use www.tinypng.org to shrink them down.

Pixelmator.....?????
What is it, a software like photoshop????

It's not clear to my why, in 2012, you'd be using an 8-bit format and dithering your colors. Why not just use PNG24?

Dave