Give it time, I assume its only been a few days? They are still getting used to each other. This will take time!

My kitty has been around my pup for months and every now and then he will let them know if they have crossed a line, or if he is just moody he will hiss at them for just being in the same room. They will get along though eventually, even if not then they will learn to tolerate each other and to stay away and leave each other alone