How do I mark a warmshowers host as "responsive" or as "unresponsive"?

At the bottom of each member's account is a box that contains the highlighted links labeled "Mark this member as responsive" and "Mark this member as Unresponsive". Clicking on either of these links will add the following statement to the bottom of their account "X members have marked [username] as [un]responsive" where X is the number of members who have made this selection.
Recording a host's responsiveness is a relatively new feature. And many guests forget to use it. So if you don't find a member marked as either responsive or unresponsive you shouldn't read too much into it.
If unsure how to access a member's account the website search tool can help you locate it. Click here for assistance on using this search function.