This topic comes up regularly -- how to use a kong, and what to fill it with! This page has some excellent ideas for using it and stuffing it, step by step.
Don't forget that cavaliers do not need that much food so a small or medium kong is the right size and consider feeding your dog's meal using the kong or using the lower cal options. You can even just smear some peanut butter on the inside -- a smallish amount! -- and keep a cavalier busy for hours...
Note there are some game ideas here too.
And remember that dogs can take time to figure out kongs... or to find a filling that interests them. Almost all dogs really enjoy them but need a chance to explore and it may take days or weeks for them to 'get it'.
They are one of the very best toys to keep a dog busy, help with separation anxiety or destructive boredom, etc. :D
I have found that a good organic peanut butter with no sugar or salt inside the kong keeps Charlie amused for quite a while.
There's a brand of treats called "Northern Biscuits" and they fit PERFECTLY snug into the kong. I usually put in a couple of the zukes super green's treats, then break a biscuit in half and put one half in each kong to trap the smaller treats in until they can get the biscuit out. Keeps the girls occupied for about 5-10 minutes and makes cleaning the kong A LOT easier (I dont have a dishwasher...).