Which Words Should I Count?
For this purpose [Appraisal is different] we are looking for the total number of words from the start of the body of the script ("Act 1, Scene 1" or the equivalent thereof) through to the final curtain.
This includes all the lines of speech (and the speaker's name at the start of the line) and all stage directions.
 
How do I find the word count of my script?
Microsoft Word:-
Word will give a word count for a selected section of text. 
Move the cursor to the start of the text to be counted.
Hold down the Ctrl key whilst scrolling to the end of the text.
That should highlight the text.
In recent versions of word, the word count appears in the bottom left hand corner of the window.
In older versions, from the menus, [Tools] [WordCount...] will tell you the count (and a bit more besides).
 
Open Office
Open Office works in a similar way to (older versions of) Word.
 
Other Word Processors:-
We don't know everything.  You'll have to get help from somewhere else.  Try the word processor's Help system or manual!
 
Isn't the number of pages enough?
No.
Page count depends on line spacing, average line length, font size, and page margins, amongst other things.  Word count is the best guide to the amount of work we will need to do (and, incidentally, also the best guide to the running time of a script - 10 000 words on the page will run to about an hour on the stage.)