Zi will move it if he’s not happy with where it is
When you unzipped the MODx package, in the Install folder you will find .tpl files with all of the default snippets. Just create a new snippet in the Manager, and copy/paste the text from the .tpl file you want. The names are case-sensitive; it has to be the same when you call the snippet. The contact form is snippet.contactform.tpl.
If the snippet’s already installed, you don’t have to do that.
Now, in the document you’re using as the Contact Us page, just call the ContactForm snippet:
[[ContactForm? &sendTo=`[(emailsender)]`]]