Can't click the checkbox to select the SimpleTest module

This is an odd problem. I am trying to enable e-commerce functionality, and, as a side issue, the SimpleTest module. I have followed these instructions from install.txt file in the simpletest module directory:

1. Add the following code to the bottom of your site's settings.php file. If
you are running Windows make sure you encode the file with the UNIX
standard.

$GLOBALS['simpletest_installed'] = TRUE;
if (preg_match("/^simpletest\d+$/", $_SERVER['HTTP_USER_AGENT'])) {
$db_prefix = $_SERVER['HTTP_USER_AGENT'];
}

2. (Optional)
It is hightly recommonded, but not neccessary, that you add the following
code to install.php at the beginning of install_main(). Should look like
7.x branch of Drupal core.

// The user agent header is used to pass a database prefix in the request when
// running tests. However, for security reasons, it is imperative that no
// installation be permitted using such a prefix.
if (preg_match("/^simpletest\d+$/", $_SERVER['HTTP_USER_AGENT'])) {
header($_SERVER['SERVER_PROTOCOL'] . ' 403 Forbidden');
exit;
}

Everything seemed fine but, when I go to Administer -> Modules, and try to click the Enable check box to enable SimpleTest, I can't do it. I "click" the box and nothing happens. I am able to select and unuselect other modules on the page. There is no visual indication that anything is wrong.

Has anyone seen behavior like this? Thanks in advance for any advice you can give.