Add link to registration form

All the custom fields labels can be translated.
You could add something like below in the override zone of your translation file (that you can find in the config under the tab languages when editing a translation):

ACCEPT_EULA="Accept <a target='_blank' href='LINK_TO_EULA'>EULA</a>"

That will open in a new window. If you want to make it a popup, that's a bit more complicated as you first need to load the modal.js library of joomla on that page (in your template maybe ?) and then you can use something like that:
ACCEPT_EULA="Accept <a class='modal' href='LINK_TO_EULA'>EULA</a>"

That HTML code is not valid. You need to do like that:

ACCEPT_EULA="<a href='#' onclick='window.open(\'index.php\',\'NewWindow\',\'toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=no,copyhistory=no,width=400,height=400\')'>view</a> and<br />accept EULA"

Could you do a screenshot of the problem ?

If you edit your custom field, you should be able to customize the error message in order to display what you want when the checkbox is not ticked.

Ah yes, you need to change the code:
allTypes = new Array('multivalues');
to:
allTypes = new Array('multivalues','required');

in the file administrator/components/com_hikashop/types/fields.php