First you might wanne know we have a dutch xupport site... :)
Second, you can try this, I'm not completely shure what exact you want to reach since you did not closed all tags correctly...
<{if $xoops_isAdmin == true}>
<tr>
<td colspan="2" id="headerbar" align="left">|<a href="<{$xoops_url}>/admin.php">Admin sectie!</a>|</td>
</tr>
<{/if}>
<{if $xoops_isuser == false}>
<tr>
<td colspan="2" id="headerbar" align="right">|<a href="<{$xoops_url}>/user.php">Inloggen</a>|</td>
</tr>
<{else}>
<tr>
<td colspan="2" id="headerbar" align="right">| Welkom <{$xoops_uname}> |</td>
</tr>
<{/if}>