OK, I found the answer in another post, sorry.
As mentionned in the installation guide, you have first to copy the JA mesolite layout in the right and select this layout. I copy the installation note :
Step 3: Go to Extension folder and extract ja_vm_themes.zip
Step 4: Upload extracted file to your Joomla_site_path. It is right now, if you see folder ja-mesolite in Joomla_site_path/components/com_virtuemart/themes/
Step 5: Go to Joomla! Admin>>Components>>VirtueMart>>Configuration>>Site: in Layout frame, choose ja-mesolite in dropdown list Select the theme for your shop
And then it works fine !