Check out "How to use Script (eg. JavaScript etc.)":
FIRST, some info from config-default.php:
//$filterxsshtml
// Enables filtering of suspicious html tags in survey, group, questions
// and answer texts in the administration interface
// Only set this to false if you absolutely trust the users
// you created for the administration of LimeSurvey and if you want to
// allow these users to be able to use Javascript etc. .
$filterxsshtml = true;
Follow these steps:
In config.php add $filterxsshtml = false; (maybe as a last line before ?>

.