1/ Install the block like other blocks
2/ Visit the Administration > Site administration > Appearance > Additionnal HTML (MY_MOODLE_SITE.com/admin/settings.php?section=additionalhtml)
3/ Adapt this code (change MY_MOODLE_SITE.com with your domain) and paste it into the « Within HEAD » area :

<!-- Dyslexic mod -->
<link rel="stylesheet" type="text/css" href="MY_MOODLE_SITE.com/blocks/dyslexic/opendyslexic.css">
<script src="MY_MOODLE_SITE.com/blocks/dyslexic/stylechange.js"></script>
<!-- End of Dyslexic mod -->

4/ Insert the block into your site.