You need to make sure that whatever you use does not have preservatives in it. It will stop, or at least hurt your fermentation. I have been using fruit, but will be experimenting next year with concentrates. When using fresh fruit: freeze the fruit then thaw it before adding to the secondary. This will help break down the cells and assist the yeast with fermentation. If you want to try concentrates/purees/extracts: try your local Home Brew Shop. They should have a good selection for you to choose from.
As far as when to add, I always add fruits to the secondary, but that is just what I've been doing so far. There is no right or wrong way. I've heard that adding fruit to the primary will lead to a weaker fruit taste as most of the flavors and aromas will go out the airlock during primary fermentation. For this reason, I've always thought it a waste. But recently we've found some references at the BJCP Website
that talk about adding fruit to the primary and the benefits of doing so. We are going to do some experiments with that, but I am not sure how that is going to work out yet.
A really great reference for you would be Schramm's The Complete Meadmaker. He goes into some nice detail concerning how much fruit to add and when.