If you’re using cPanel, usually a “right-click > change permissions” will bring up the file permissions dialog, but I think your current .htaccess file actually looks correct if you are using the /%postname%/ option in your custom permalinks. The file must already be writable to your user account, so that may not be the issue.
Without changing the file permissions, you might try a couple of things:
– Go the dashboard and reset the permalinks to default, and save the changes: See if the 404 goes away. If it does, reset the permalinks to your custom preference and see if that fixes the issue. If not;
– Take a look at the menu item you created for the Contact Us page, and make sure it points to the correct url. I noticed that the page title is Contact Us, but the slug on rollover says “Contact Me”, so that’s something else to look at.
– Also, you don’t have any other content in the way of posts, categories, tags, etc.. so be sure that your “Front page displays” settings are the way you want them. If you have a posts page defined (and it happens to be the Contact Us page) I think that could also cause a 404 if there aren’t any actual posts to display.