Intall it just as regular extension:

1. Upload .ocmod.zip file via extension installer

2. Refresh Modification

3. Enable google feed extension.

4. Then modify .htacesss

# Add the following line after Rewrite Engine statement

RewriteRule ^sitemap.xml$ index.php?route=extension/feed/google_sitemap [L]
RewriteRule ^product-sitemap.xml$ index.php?route=extension/feed/google_sitemap/product [L]
RewriteRule ^category-sitemap.xml$ index.php?route=extension/feed/google_sitemap/category [L]
RewriteRule ^information-sitemap.xml$ index.php?route=extension/feed/google_sitemap/information [L]
RewriteRule ^blog-sitemap.xml$ index.php?route=extension/feed/google_sitemap/blog [L]
RewriteRule ^faq-sitemap.xml$ index.php?route=extension/feed/google_sitemap/faq [L]