I have a spaceship that I want to have "flames" come out when the spacebar is pressed. I have set up a replace mesh thing (I have a mesh with flames in a different layer and added some logic bricks to replace it when the spacebar is pressed), but for some reason, I can't select the "Spaceship with flames" mesh on the logic brick. This is the mesh I want to have replace the spaceship.
Be aware object names are not mesh names!
(Blender) objects and (Blender) meshes are two different entities. They can but do not need to have the same name.
The replace mesh actuator allows you to search for meshes by (mesh name) within the same scene.
I suggest you select your flame object and check the mesh name at properties/mesh