vb condition in a forum block

[deverill2010]

How would I go about getting the following to work in a forum block? As vb conditions don't seem to work how i've written it.

<center><a href="http://www.midwiferyonline.co.uk/store/index.php/midwife-in-the-making-hoodie-7/hoodies.html">
<img src="http://www.midwiferyonline.co.uk/images/ads/members10discount.jpg" alt="10% OFF Hoodies" width="180" height="220"></a>
<br /><br />
<vb:if condition="$show['guest']">
<a href="http://www.midwiferyonline.co.uk/register.php">Register</a> or <a href="http://www.midwiferyonline.co.uk/login.php?do=login">Login</a>
<vb:else />
<img src="http://www.midwiferyonline.co.uk/images/ads/members10discountcoupon.jpg" alt="10% OFF Hoodies" width="155" height="166"></a></vb:if></center>