If you just want a stock contact form, you might try SPForm, which will autoinstall a working, spam-proof form for you. Make sure you don't already have a contact or thank-you page before installing it (just rename both and their aliases).
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODx info for everyone:
http://bobsguides.com/modx.html