I have read on many websites that it is not safe to put frozen meat in a slow cooker because it does not come up to temperature fast enough and therefore can produce foodborne bacteria in it and make you sick. Here is a website that talks about it and down towards the bottom of the article are the resources of her information.
http://www.slowcookercentral.com/slo...-its-not-safe/
In the oven I cook at 250 degrees, which is a much higher temperature than a slow cooker.