It's rather annoying that this Addon does not include any Readme.txt
For that reason alone, I'm sure it would be rejected if it were submitted today.
I had to read its catalog files to discover how to view the pulsar.
The pulsar in that addon is defined as a Planet orbiting the Star HIP 333340.
To see it, first select the star, then select the pulsar, then you can goto it.
For example, after starting Celestia, you can type the following:
In the above sequence of commands,
[enter] means to type the large Enter key which is just to the right of the main part of your keyboard
g means to type the g key to GoTo the selected object.
It'll be a little hard to see against the bright nebula behind it, but if you rotate the field of view, it'll show up better against the blackness to the side.
Does this work for you?