You can't insert PHP into a post or page with this plugin, sorry.
As far as I know, there are no plugins that can do this. You could try to modify my plugin for this purpose, but the results will probably be unexpected.
Try changing the following:
Line 93 through 95 of perpagehead.php:
should be changed to:
This will cause EVERYTHING you put in the box to be evaluated as PHP (as if it was inside <?php ?> tags , so use ?> HTML here <?php to put html in). However, using the eval() function is usually a bad idea, I haven't tried it, and won't support it. Also, this causes non-admins to be able to execute PHP code on your webserver, so it seriously compromises security (if you can execute PHP, you can empty the SQL database, delete and modify users, promote yourself to admin, take over the site etc.).
To be short: this is a very bad idea, you shouldn't do this, but if you still want to, this is the way.