Hi dougal.

You have to have the Acceleration Expansion Pack because both XLoad aircraft use the VC from the Acceleration F/A-18. Without it, no VC.
From the System requirements page for XLoad:

MINIMUM SYSTEM REQUIREMENTS:

   * - PC with 1.5 GHz equivalent or higher processor
   * - 256 MB of system RAM
   * - 64 MB DirectX 9 compatible video card
   * - 500 Mb of free space on the MS FS hard drive
   * - Microsoft® Windows® XP SP2 / Vista
   * - Microsoft® Flight Simulator X Acceleration*
   * - Internet access (for installation/ re-installation)


Please note, we are unable to support technical issues appeared on systems that do not conform to the minimum system requirements.

* - XLoad can be installed on Microsoft® Flight Simulator X. But the following limitations apply:

     - No FSX Acceleration F/A-18 sound
     - No FSX Acceleration F/A-18 VC
     - No FSX Acceleration F/A-18 2D Panels

Mark