Andate in Pannello admin->Stili e Template->Gestione stili->Modifica template->Template navigazione->Navbar.
Trova:
<strong><phrase 1="$bbuserinfo[username]" 2="member.php?$session[sessionurl]u=$bbuserinfo[userid]">$vbphrase[welcome_x_link_y]</phrase></strong><br />
e sostituisci con:
<strong><phrase 1="$bbuserinfo[musername]" 2="member.php?$session[sessionurl]u=$bbuserinfo[userid]">$vbphrase[welcome_x_link_y]</phrase></strong><br />