Error upon activation

I installed this plugin, and when I tried to activate I got this error:

Cannot redeclare the_excerpt_max_charlength() (previously declared in /home/user/public_html/wp-content/themes/mytheme/functions.php:400) in /home/user/public_html/wp-content/plugins/facebook-revised-open-graph-meta-tag/index.php on line 290

http://ww.wp.xz.cn/plugins/facebook-revised-open-graph-meta-tag/