clevererror plugin

I’m simply trying to edit the 404 plugin “CleverError” on my website…http://improvedconstructionmethods.com. The 404 page looks decent, but I want to add some of my own wording, etc. I go to the Plug-in page on WP, then go to the CleverError plugin and then click “edit.” I do not see the necessary wording listed to make changes. If I go to the “Page Source” of my actual browser, I do see the wording. This does not make sense to me. Shouldn’t everything that appears on the Page Source show up under the WP editor? Any suggestions/help is greatly appreciated! Again, this is for the plugin-called CleverError. One more thing…are there any other 404 plugins you recommend? I want to have a really cool 404 page – not simply redirect to another page. Thx!!!!!