I too am under the impression you CAN NOT TRADE SHORT IN A RETIREMENT ACCOUNT.
The idea is with limited funds the stock could potentially run against your trade and you would have to cover with a margin call. You can NOT HAVE MARGIN IN A RETIREMENT ACCOUNT either. You are held to a limited amount of $'s added to a retiremnt account yearly unless it is a KEOG in which case you are limited to 25% of your net business gains(still here it is capped)