Here is a step by step example for writing 103P/Hartley 2 data directly to the Comets.txt files stored in the Sky Data folder.  You can add single or multiple comets using this method.

1.  Go to http://www.minorplanetcenter.net/iau/MPEph/MPEph.html

2.  Enter 103P/Hartley 2 into the search box.  

Make sure under "Format for elements output:" you select "Starry Night (Sienna Software)":

3.  Click on 

4.  An elements.txt will be downloaded to your computer.

5.  Open the elements.txt file and you will see the following:

6.  Open the Starry Night comets.txt file, add the new line of data, and save.

Note, I have changed its name from Hartley to 103P/Hartley 2.

The comets.txt file is located in the following locations:

Windows: (C:) ---> ProgramFiles ---> Starry Night folder ---> SkyData ---> Comets.txt

Mac OSX: Macintosh HD ---> Applications ---> Starry Night folder ---> right click the Starry Night program icon ---> Choose "Show package contents" ---> Contents ---> Resources ---> Sky Data ---> Comets.txt

7.  Start Starry Night.  Open the FIND tab.  Search for "103P" (begins with) and the comet will show up:

Minor Planet Center's orbital elements for 103P/Hartley 2 are attached (April 2012) if you wish to use them.