DX plugin support is available in the 32-bit Windows version only.
DX (DirectX) plugins can be loaded in an effect slot using the slot's down arrow. The DX plugins appear in the effect selector's "VST and DX plugins" section. The plugin will be shown in a window with Bypass and Presets buttons.
MultitrackStudio automatically detects whether the plugin supports mono in to stereo out conversion (reverbs usually do). Some plugins do not support this, if this is the case you can force the plug in to operate in stereo in to stereo out mode by adding a Stereo Effect just before the DX plugin.
Note: if you install DX plugins while MultitrackStudio is running you may have to restart MultitrackStudio in order for the new plugin to be listed (the plugins are collected only once).