Uhm... I suspect you do not know how to get into directories using the terminal... Is very easy (probably is harder to say how to do it than actually do it...). We have to use the command "cd" to go where we want in the filesytem. You said that you unpacked the celtx archive onto desktop. So:
in a terminal will put us into the desktop folder. You will notice the change of directory by the change of the string of the prompt. You can also type
to view the directory content.
Then, assuming that the unpacked celtx folder is called celtx (as I guess watching your reposts):
to enter the directory. Remeber that everything in linux is case sensitive, so typing Celtx is different from typing celtx. As above the command ls will give to you the directory content. The executable files should be green coloured. If the executable is called celtx then we have to run
and the terminal will give to us information about the program launching.
As I'm not so good at explain myself, I'll do an example:
You see: there is a directory called Folder, then:
user@computer:~$ cd Desktop
AnalogHarmonizer.ZIP Folder SuperOctaver.ZIP
EH-Guitar-Synth.ZIP Guitar Synth.pdf
If you need help type man followed by the command name, for example:
user@computer:~/Desktop$ cd Folder/