Excerpt and MORE doesnt work

I cant seem to get either the MORE tag working, or the optional excerpt to appear.

When a user clicks on a category in my blog they get the full articles all on one page, and I need to split them to just a summary, then a click through to the full post.

I have tried inserting the MORE tag, but this makes no difference, neither does writing a summary in the excerpt box when writing a post.

What do I need to do, and to what template page to make either of these things work?

Thanks