Hi,
First of all, i should make things clear;
- LightMart is a Joomla Template (with Virtuemart Theme integrated).
- PureMart is a Virtuemart Template (Virtuemart Theme only).
So, you can't use LightMart along with your existing joomla template, because LightMart is already a Joomla Template. Joomla can have only 1 template at a time.
LightMart can be installed through "Extensions Manager" because it is a joomla template. Don't try to upload any files to anywhere manually. If you perform the one-by one method, then you have to install all the ZIP files (by following the TUTORIAL) and then select the "LightMart" as your "default" template through (joomla backend) Template Manager. Then you will start using LightMart in your joomla but you would lose your current joomla template (which is rt_afterburner2 in your case).
On the other hand, PureMart is a Virtuemart Theme, and can be integrated to any joomla template by uploading the necessary files to
ROOT/rt_afterburner2/html/com_virtuemart
Which i guess you are already familiar with that.
If you are getting SAVE FAILED when you try to save the "username" and "order-id" for LightMart Joomla Template, then try to use a different browser (IE, SAFARI or CHROME). it is a known and browser related issue. Hopefully this problem will be solved soon in the next version. The "check update" link was supposed to go to this page:
www.flexiblewebdesign.com/forum-joomla-virtuemart-template-lightmart-support/969-request-lightmart-update
Regards.