How do I join in with Birthday Cakes and all the critters that visit? I can't find Birthday Cakes in the feed store and I've noticed that some have 5 year cakes and 15 year cakes.

Marlee you need have an empty feeding table in your garden. Then you go hunting to spot birthday field mice. When you spot your fifth or multiple of 5 it will drop birthday cake onto your empty table. Check out the birthday wildlife section. Other birthday wildlife will also drop cake after 2 spotted etc.

Just to add to what Judith said:

The two types of birthday cake are 10th and 15th.  Some only attract 10th birthday wildlife, some only attract 15th wildlife, and some attract both.  You can see which does which, and that's also listed against each animal in the Birthday section of the Wildlife Guide.

For the ones which drop both (birthday field mice, WRC and mermaids) they will drop the type of cake they were spotted on.  So if you want 15th birthday cake, for instance, you'd either need to hunt for wildlife which only drops 15th cake, or you'd need to make sure that the one you spot which would drop cake (every multiple of 5 mice, 3 WRC, 5 mermaids) was feeding on the 15th cake otherwise it would drop 10th cake.