Fatal Error when I activate plugin…please help!

I installed Q&A plugin; checked my Permalinks (I am using /%postname% for Permalinks setting).

Inserted the Questions List widget into my sidebar and my site crashed with a fatal error:

Error Code: php_code_error:1:disappointed:home/elucia/public_html/blog/wp-content/plugins/qa/core/functions.php:24:Call to undefined function is_post_type_archive()

Message: A fatal code error occurred.

there is more to the error code referencing my server/directories but did not want to post.

the website is here: http://e-girlfriday.com

I am using WordPress 3.1., the theme is Builder core/Scooter Child Theme

I really want to use this plugin; any help greatly appreciated.

edit: I’ve de-activated and re-activated several times. The site crashes when I activate the plugin.

pps: do I need to set the Admin permissions? At one point I had checked ALL of the boxes in the Settings page, but I wonder if that was necessary? perhaps that caused the crash? I unchecked all of them (which I think is the Default)