In this case you don't need DESTINATION_APP, use LOOKUP to get data from model 2:
Script in ANNUAL_PLAN:
*LOOKUP MODEL2 //the real name of model 2
*DIM PM:COST_UNIT="PACK_MAT"
*ENDLOOKUP
*XDIM_MEMBERSET ACCOUNT=SALESVOL
*WHEN ACCOUNT
*IS *
*REC(EXPRESSION=%VALUE%*LOOKUP(PM),ACCOUNT=GROSSCOST)
*ENDWHEN
Vadim