If the 3rd party dll exists in the bin folder of your project after building it then you should add a line to your post build event to copy the 3rd party dll to the Web/bin folder. If its in the GAC on your machine then you may need to add a reference to your 3rd party library right on the mjoPortal.Web project to make it work, or you may need to add an assembly reference in Web.config. Probably the 3rd party provides some documentation about integrating and distributing their dll with your app.
Hope it helps,
Joe