I'm not Vernon's authority on swimming, but I would not recommend the Tempo trainer to someone who needs stroke improvemnt work ... I'm speaking generally ...
I'm sure Andrew will comment but forcing a specific cadence will be at a detriment to your technique. I have a tempo trainer and I will not use it until I am more comfortable with my stroke.
The company is Finnis and the trade name is tempo trainer (
http://www.finisinc.com/Technology/Tempo/details.aspx)
SC