ok the training is calculated every friday morning (depending on where you are in the world). You cannot change this.
It works by setting a training schedule in the training page and then playing the players you want trained in the positions that you have selected in the training page.
i.e. Training Inside Scoring for C (center) and playing an 18 year old you want to train as a center for over 48 minutes during the week (Saturday; Tuesday; Thursday = in that order)
hope you get the idea.
another little thing to note is that after your scrimmage or cup game on the Thursday it is possible to look at Weekly stats and change the training to suit the minutes that were actually played without any negative effects.