If you can see the dimensions listed in the Dimensions window in the upper left (above the Measures), and they aren't appearing when you are creating a new calculation, this sounds like either abug or a problem with your install, you might contact tech support.
The dimensions are not appearing in your calculated field box because you are connected to a cube. With cubes, dimensions cannot be used in calculated fields. As an alternative, calculated members may be able to be created. The following link provides further information:
Hope this helps!