If that doesn't work, you can try using something like
model="..\AddOns(or wherever the addon is stored)\pboname\ModelName.p3d";
The ..\ should
direct OFP to start looking for that path from the OFP root folder..
In configs the model path is 'defaulted' into the Data3D if no other path is defined OFP searches the model there, in missions the default path is always the mission root, that's why you can run scripts etc. just with exec "script.name"
instead of exec "path\scriptname.sqs"..