You should have received some code from Mediaplex that looks like the following:
<script language="JavaScript1.1" src="http://altfarm.mediaplex.com/ad/js/3178-13924-1843-0?mpt=[CACHEBUSTER]&mpvc="></script>
<noscript>
<a href="http://altfarm.mediaplex.com/ad/ck/3178-13924-1843-0?mpt=[CACHEBUSTER]">
<img src="http://altfarm.mediaplex.com/ad/bn/3178-13924-1843-0?mpt=[CACHEBUSTER]" alt="Click here for reservations" border="0">
</a>
</noscript>
What you need to do is modify the URLs in the <SCRIPT> and <A> tags as shown by the highlighted changes:
<script language="JavaScript1.1" src="http://altfarm.mediaplex.com/ad/js/3178-13924-1843-0?mpt=[CACHEBUSTER]&mpvc=%%CLICKTAG%%"></script>
<noscript>
<a href="%%CLICKLINK%%http://altfarm.mediaplex.com/ad/ck/3178-13924-1843-0?mpt=[CACHEBUSTER]">
<img src="http://altfarm.mediaplex.com/ad/bn/3178-13924-1843-0?mpt=[CACHEBUSTER]" alt="Click here for reservations" border="0">
</a>
</noscript>