After activating the plugin go to the settings page.  At the top of the page you will see a box titled Default Dimensions.  Set the default width and height for your captcha.




The second box is titled Request Key.  This is where you set the argument that will be added to a url when requesting a captcha image.  Set it to whatever string you want or leave it with the default.




The third box is titled Colors. This is where you will set the various colors for your captcha.  Click on the colored box to bring up the color picker.




The fourth box is titled General Settings.  Below is a short explanation of the settings in this box.


Preserve Settings On Deactivation:  If the plugin is deactivated all the settings you have set will be erased unless this box is checked.  This includes any uploaded font files.
Random Font Count:  When generating the captcha string the plugin randomly picks a font to use when generating a character.  An array of font files is created with this many positions for the plugin to choose from.
Gradient Transitions:  The number of transitions between the two colors set in the colors box.
Case Sensitive:  Should the string be case sensitive when comparing the users input to it.
Add To Comment Form:  Should a captcha be added to the comment form?
Add To Registration Form:  Should a captcha be added to the registration form?
Add jQuery To Header:  Some themes do not contain jquery.  Check this box to add jquery to the header.




The fifth box is titled Fonts.  This is where you will manage the different font files.  You can use any TrueType font file you want.