You can do either. You can put just the filling as long as you put a buttercream dam around it. It doesn't make the cake soggy 
Or you can fold it in to the buttercream or the bettercream and use it like that. Just depends on how string of a flavor you would like!
These fillings are intended to be used just like that. I have used them many times and they do not make the cake "soggy". I know that you can also mix some in your BC or betercreme and apply that as your filling too. I have not tried it that way, but I have heard from others that it tastes really good!! I know that CC'er Indydebi uses these fillings and she has nothing but good comments about them. They are convenient and also taste super good 
And the most important thing....they are inexpensive and can last quite some time
Thanks for the replies. Now if you use ust the sleeve do you use it on all layers or do you mix in something else on a layer? I have to make a cake for this weekend and the requested chocolate cake and told me to take it from there. I know the guy likes raspberry. I will have 2 sheet cakes on top of each other. I think torting a cake makes it even better,but I dont want it to shift under the weight of the cake and fondant. Any suggestions?
When using the sleeve how much do you put on a layer? Should it be thick or just enough to see it?
TIA
It varies on how much filling you want, once you put it in, use your judgement you will know how much you need and if you art torting each layer, you don't need a lot I use the sleeve fillings all the time and I put a pretty decent amount without any problems, and Yes you can use it on all the layers, just make sure you use a stiff buttercream dam to support the filling
another helpful hint that i have gotten is that if you use the sleeve filling and don't mix it with BC, make sure that your dam around the edge comes up higher than your layer of fruit filling. that way your top layer rests more on the BC than the fruit and you shouldn't have an issue with the layers sliding against each other. hth!
I just did a cake today with a strawberry filling (I made it myself), and I was worried about it soaking into the cake. I simply applied a VERY thin layer of BC on the cake to act as a barrier. I also did the very firm icing dam as mentioned above. No problems - yay!!
As to the amount, I use one sleeve for a 16x18 surface or 1/2 sheet cake. First, I smooth on a very thin layer of buttercream then I run a dam with stiff bc icing using a no. 12 tip and then spread on the filling.
When doing a layer cake and torting the layers I use a little less filling because you have the 3 layers of filling.
Thanks everyone. I am going to make a camper cake for Fri. It will be a bit higher than normal. I am going to make a sheet cake and cut it into thirds. (I think, I first have to see how high it is with 2 layers) I am going to have to do 1 layer, filling, layer, filling and then put a cake board in with dowels?? and then the follwoing layers and I am covering it with fondant. So do you think I will have any problems with the cake sliding? Is this cake board just a white cake board or do I have to have the hard plastic ones?
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