Running a MODX version that old is EXTREMELY risky. They need to upgrade that site as soon as possible, as it's only a matter of time before it's exploited (if not already). It's also possible the older version will have issues functioning properly if the server has been upgraded beyond what that release supported.
In regards to your actual question regarding adding a form, the tutorials that mrhaw provided should get you going. The specific page you will want (skipping ahead) is for eform (
http://codingpad.maryspad.com/2009/05/31/building-a-website-with-modx-for-newbies-part-9-adding-a-contact-form-using-eform/) which is the form processor for Evolution/0.9.x.
That being said, a faster method if you are familiar with PHP form processors is using a generic one instead. You aren't required to use MODX for handling the form processing, granted I find it's a more secure option. In this case you would just add a form handler say formprocess.php in your webroot and then for the form post value set it to that earlier file.
In MODX you would just need to create a Chunk containing the forms HTML code and add it to the contact us page. Depending on how the site was originally built this may be quick 'n dirty, but should get your job done.
Getting back to the update, if you don't feel confident enough performing the update, you might consider hiring someone to perform the update for you. They can also troubleshoot the common post upgrade problems jumping across that many versions. I still work with MODX Evolution a lot, so if you need help with this please feel free to ask.
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