I don't think that was the issue but the thread you are referring to did help me find the answer. the code for the NEW button was originally:
<span id="hikashop_checkout_<?php echo $this->type;?>_address_new" class="hikashop_checkout_<?php echo $this->type;?>_address_new">
<a class="modal" id="hikashop_checkout_<?php echo $this->type;?>_address_new_link" rel="{handler: 'iframe', size: {x: 450, y: 480}}" href="<?php echo hikashop::completeLink('address&redirect=checkout&task=add&step='.$this->step,true);?>" onclick="SqueezeBox.fromElement(this,{parse: 'rel'});return false;">
<?php echo $this->cart->displayButton(JText::_('NEW'),'new',$this->params,'','var link = document.getElementById(\'hikashop_checkout_'. $this->type.'_address_new_link\'); if(link) SqueezeBox.fromElement(link,{parse: \'rel\'});return false;'); ?>
</a>
</span>
I had a look at the Edit button to try and find out what was different and I noticed it had no id= but rather class= whatever with a space in between and then the content from id so I changed the new button to be the same and it works now:
<span id="hikashop_checkout_<?php echo $this->type;?>_address_new" class="hikashop_checkout_<?php echo $this->type;?>_address_new">
<a class="modal hikashop_checkout_<?php echo $this->type;?>_address_new_link" rel="{handler: 'iframe', size: {x: 450, y: 480}}" href="<?php echo hikashop::completeLink('address&redirect=checkout&task=add&step='.$this->step,true);?>" onclick="SqueezeBox.fromElement(this,{parse: 'rel'});return false;">
<?php echo $this->cart->displayButton(JText::_('NEW'),'new',$this->params,'','var link = document.getElementById(\'hikashop_checkout_'. $this->type.'_address_new_link\'); if(link) SqueezeBox.fromElement(link,{parse: \'rel\'});return false;'); ?>
</a>
</span>
I hope what i've done will not break anything else?