I agreed with andre, in AX 2012, you can not find main account as string value in transaction table. if you go to SubledgerJournalAccountEntry table , you will get ledgerDimension field where you could get main account with segmented entry value.
Ledger dimension is nothing but int64 field. if you want fetch the main account from ledgerDimension please use below API for same
\Classes\AxdDimensionUtil\getLedgerAccountValue( ledgerDimension ), it will return the container from where you can find the ledger account.