If using the source code of mojoPortal, Microsoft.Web.Preview.dll is included in the _libs folder and when you build it should get copied to the Web/bin folder. First check if it exists in Web/bin, if not copy it there.
If it does already exist then the problem may be that you have an older version installed in the GAC on your machine and this older version is interfering so you need to uninstall it. This could happen from an old install of Ajax.
Hope it helps,