[Lynne]

You need to tell us what template you plan to use this in in order for use to help with the variable name. $userinfo is probably correct on, say, the member page, but in the showthread page you will want to use $post.

<if condition="$userinfo['field16']==Yes">$userinfo[field15]<else />$userinfo[username]</if>

Also, usually musername is used, not username. But, again it depends on the template you are wanting to put this code in. Find the template and then find the code for the username in it and post that here if you need more help.