You can put $navbar_advertisement in any template anywhere you like, it is a global advertisement.
If you want to style/position it, edit the navbar_advertisement
template to your taste. If you want it centered, change <div> to <div style="text-align:center;">
To put it in your navbar to the right (this is untested, but it's roughly the right approach if not quite correct) look in your navbar template for the following code and note the change I made which are highlighted in bold.
Code:
<td class="alt2" valign="top" nowrap="nowrap">
<div class="smallfont">
<strong><phrase 1="$bbuserinfo[username]">$vbphrase[welcome_x]</phrase></strong><br />
<phrase 1="$pmbox[lastvisitdate]" 2="$pmbox[lastvisittime]">$vbphrase[last_visited_x_at_y]</phrase>
<if condition="$show['pmstats']"><br /><phrase 1="$vbphrase[unread_x_nav_compiled]" 2="$vbphrase[total_x_nav_compiled]" 3="$session[sessionurl_q]">$vbphrase[private_messages_nav]</phrase></if>
<if condition="$show['pmwarning']"><br /><strong><phrase 1="$vbphrase[pmpercent_nav_compiled]">$vbphrase[your_pm_box_is_x_full]</phrase></strong></if>
</div>
</td>
<td class="alt2" valign="top" nowrap="nowrap">
$navbar_advertisement
</td>
<else />
<td class="alt2" nowrap="nowrap" style="padding:0px">
<!-- login form -->
<form action="login.php?do=login" method="post" onsubmit="md5hash(vb_login_password, vb_login_md5password, vb_login_md5password_utf, $show[nopasswordempty])">
<script type="text/javascript" src="clientscript/vbulletin_md5.js?v=$vboptions[simpleversion]"></script>
<table cellpadding="0" cellspacing="$stylevar[formspacer]" border="0">
<tr>
<td class="smallfont"><label for="navbar_username">$vbphrase[username]</label></td>
<td><input type="text" class="bginput" style="font-size: 11px" name="vb_login_username" id="navbar_username" size="10" accesskey="u" tabindex="101" value="$vbphrase[username]" onfocus="if (this.value == '$vbphrase[username]') this.value = '';" /></td>
<td class="smallfont" colspan="2" nowrap="nowrap"><label for="cb_cookieuser_navbar"><input type="checkbox" name="cookieuser" value="1" tabindex="103" id="cb_cookieuser_navbar" accesskey="c" />$vbphrase[remember_me]</label></td>
</tr>
<tr>
<td class="smallfont"><label for="navbar_password">$vbphrase[password]</label></td>
<td><input type="password" class="bginput" style="font-size: 11px" name="vb_login_password" id="navbar_password" size="10" tabindex="102" /></td>
<td><input type="submit" class="button" value="$vbphrase[log_in]" tabindex="104" title="$vbphrase[enter_username_to_login_or_register]" accesskey="s" /></td>
</tr>
</table>
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="hidden" name="do" value="login" />
<input type="hidden" name="vb_login_md5password" />
<input type="hidden" name="vb_login_md5password_utf" />
</form>
<!-- / login form -->
</td>
<td class="alt2" valign="top" nowrap="nowrap">
$navbar_advertisement
</td>
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