In order to animate textures in Sandbox, you need to have a particular name in the material editor like this.
For a hands-on example please download this sample package animate_test.rar.
.../number##ns_necount(0.5).dds
Here is an example of the texture in the material editor: objects/number##00_04count(0.5).dds
Then the actual textures should have the following names: number00count.tif, number01count.tif, ..., number05count.tif.
It's also possible to use the format: number00##. In this case the engine will try to load all textures from name number0000 until the last texture file that's available.
Example texture files