There are two problems here. The first is that you are not getting any fog glow and this is easy to resolve - the problem is that only parts of the image that are above the Threshold setting of the Fog Glow node will be made to glow. As you appear to not have any bright emission parts of your render, using a Threshold of 1.0 will result in no glow - try reducing the Threshold in order to see the glow.
As for the separating the glow so as to only affect certain objects in the scene, you can use the Object Index render pass to generate a mask in order to affect only those elements that you require to 'glow'. In order to do this, first give each of your objects that you require to glow a specific Object Index by setting the Pass Index in the Object properties in the right-hand panel :
Enable the Object Index pass by enabling it in the Render Layer :
In the Compositor Nodes, you can now use the following nodes :
Here the ID Mask is set to the same Index as you have used to identify your object(s). This will produce 1.0 for each pixel corresponding to the parts you want to 'glow' and 0.0 for the rest of the image. The subsequent Mix nodes generate two images - one where the objects matching the Index are black while the rest of the image is shown, while the other where the object is shown with the rest of the image black.
The Fog Glow can then be applied to the relevant image to apply the glow - ensure to set the Threshold to zero so as to glow everything that is greater than 'black'.
All that is now required is to recombine the two images using the Add mode - any black regions will be replaced with the other image. Since the masks used to generate the images are mutually exclusive, only the 'glow' parts of the images will overlap.
This can produce the following result :
Looking at your Blend file, you've got some minor differences in your setup that are affecting your results. This should be amended as follows :
Not the highlighted changes; the Mix nodes should be mixing black (otherwise the white will also be "glared"), the two branches should be combined with Add instead of Mix, the Fog Glow needs a lower Threshold.
Note that the Fog Glow produces quite a subtle effect. A quick way to increase it would be to chain multiple Fog Glow nodes together :