If you're looking for a particular comet in Starry Night and can't find it, here are a few things that might be keeping you from seeing it.
1) Make sure you have the latest version of the Comets.txt file.
In Starry Night 7, you can click on Help ---> Check for Data Updates on the menu bar, and choose Check for Data Updates in order to get the current version of the file.
In Starry Night 6, you can click on LiveSky ---> Check for Data Updates on the menu bar, and choose Check for Data Updates in order to get the current version of the file.
If you can't access it that way, you may also download the file manually from the following location:
Save the file in your Sky Data folder:
In Starry Night VERSION 7 the SkyData folder is here:
(Windows) \Users\<YourUserName>\AppData\Local\Simulation Curriculum\Starry Night Prefs\Sky Data
(Note that the AppData folder is often "hidden". A quick Googling can show you how to un-hide it)
(OS X) /Users/<YourUserName>/Library/Application Support/Simulation Curriculum/Starry Night Prefs/Sky Data
Note that this is where any "streamed" data will be located too.
In Starry Night VERSION 6 the SkyData folder is here:
(Windows) browse to C:/Program Files/Starry Night/Sky Data.
(OSX) in Applications, right-click or control+click your Starry Night program icon and choose Show Package Contents, and browse to Contents -> Resources -> Sky Data.
Click File/Save in your browser window when you are viewing that file, and save it into your Sky Data folder. Let it replace any older file with the same name.
2) Your program may not be set to display all the comets in the file. Click on File/Preferences... on your menu bar (or, Starry Night/Preferences, if you're using a Mac) and choose Startup/Auto Updates. Set the number of comets read in on startup to 600. This ensures that all comets in the file will be loaded into Starry Night.
3) Older versions of Starry Night may not be able to read the current comet data file, since it's bigger than it used to be. If you have a version of Starry Night that does not let you change the read-in number, you can edit the comet data file. Browse to your Sky Data folder and open the Comets.txt file in a text editor. Find the line for the comet you want (if it's a newly-discovered comet, it will be near the bottom of the file) and move it to the top of the file. Save the file and start Starry Night again, and it should force the program to load that comet first.