how can i subscribe to a podcast series from itunes on my ipod touch
As far as I know, you can't subscribe to them from your iTunes app on your iPod. 

The easiest way I know of is to subscribe to whatever podcast you want to listen to in your preferred feed reader, and then whenever that podcast is updated you'll see it in your feed. From there you can just choose to play or download it. Hopefully this helps solve your issue.