In the ShaderFX editor, connect your transparency value to the Opacity attribute of the TraditionalGameSurfaceShader shader.
To prevent your objects from rendering double-sided, you can select the object's shape node, then disable Double Sided under the Render Stats section of the Attribute Editor.
For the best transparency quality, select Renderer > Viewport 2.0 > , then select the Depth Peeling Transparency Algorithm. However, this option provides high quality with a performance cost.