This WordPress plugin will generate an XML Mobile Sitemap for your WordPress blog.
Once you have created your Sitemap, you can submit it to Google using Webmaster Tools.
You can find the sitemap-mobile.xml file in your blog's root folder.
It implies that you don't have write permissions on your blog's root folder. Please use chmod or your FTP manager to set the necessary permissions to 0666.
According to Matt Cutts of Google, you may not need a separate Mobile Sitemap if your WordPress blog is using Responsive Web Design where the same page is served to both desktop and mobile screens. Everyone else may need one.