The code you have got should work and it should be pasted anywhere on your webpage where you want the button to appear. For example, try this:
1) Create a blank webpage in DW by going to File >> New
2) in the body section paste this code:
<!-- Place this tag in your head or just before your close body tag. -->
<script type="text/javascript" src="https://apis.google.com/js/plusone.js">
{parsetags: 'explicit'}
</script>
<!-- Place this tag where you want the share button to render. -->
<div class="g-plus" data-action="share" data-width="350" data-href="http://www.mytaxsite.co.uk"></div>
<!-- Place this render call where appropriate. -->
<script type="text/javascript">gapi.plus.go();</script>
3) Save the page;
4) Preview the page in your browser.
Now this should give you the idea of what is exactly involved. "Render" simply means to supply or deliver. In this case you are rendering the "Button" supplied by Google except that you don't have it physically because it is linked directly to Google's servers.
Hope this gives you the idea to get started. This code will render a webpage that looks like this in your browser:
Click on this picture to get a magnified image:
