Display of timestamps is dependent upon which message style you're using... check your Messages preferences (and if one of the built-in ones doesn't do it for you, you might want to look at the styles available from www.adiumxtras.com)
In case you want dates in your message time stamps and you use Smooth Operator, I did some tweaks to make that available by adding a 'Icon-DateTime' variant. Source here: https://github.com/hrimhari/adium
I posted a solution to a problem I personally spent some time trying to solve hoping it would help other people. This thread appeared close to the top of my google search.