and welcome to the A2A simulation community and support forums. The issue you have is a common one that catches out new people to the sim and also some veteran simmers when re-installing. In order to use many add on's of course make sure you are up to date you will want to make sure your make sure you have the two FSX service packs or the Acceleration expansion package which includes the two service packs.
Once you install the two FSX service packs from Microsoft you will be able to use the Accu-sim C172 Trainer and most other add on aircraft that require FSX SP2.
Here's a link to the MS website;http://www.microsoft.com/Products/Games ... fault.aspx
Lewis - A2A
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