You can trade your STEEM on the BitShares Exchange using OpenLedgers Deposit and Withdraw.
First you will need a BitShares account. Start one for free here:
BitShares Exchange Sign-up and Market
If you already have an account the above link will take you to the STEEM / BTC market offered by OpenLedger.
OPEN.STEEM is STEEM held by OpenLedger for exchange
OPEN.BTC is BTC held by OpenLedger for exchange
In order to trade you will need to deposit either STEEM or BTC. The process is simple.
Using OpenLedger's wallet you can click the tab marked "Deposit/Withdraw" at the top and this will give you various exchanges to select from to make your deposit. To use OpenLedgers OPEN assets described above you will select the "OpenLedger" tab from the available choices. From here you can select Deposit or Withdraw and you will find a drop down of all available coins and instructions.
STEEM DEPOSIT
To deposit STEEM to OpenLedger you will simply send your STEEM to the steemAccount:"openledger" with a memo: "open.steem:<yourBitsharesAccount>" where <yourBitsharesAccount> is the BitShares account name where you are trying to deposit. This will automatically issue your BitShares account with OPEN.STEEM which can now be traded on the market linked above. (or any market where OPEN.STEEM is traded)
Here is an example CLI command if I were to send from my steem account "xeldal" to a BitShares Account name "example":
transfer xeldal openledger "1.000 STEEM" "open.steem:example" true
[UPDATE] Steemit.com now has a wallet. Click on your profile icon in the top right and select "Wallet" . Then click the small carrot beside your steem balance and select "transfer" and fill in the "To" and "Memo" fields as described above.
STEEM WITHDRAWAL
To Withdraw your OPEN.STEEM back to real STEEM on the Steem Network, simply send your OPEN.STEEM to bitsharesAccount:"openledger-wallet" with the memo "steem:<yourSteemAccount>" Where <yourSteemAccount> is the Steem Account name you are trying to withdraw to.
The form to send OPEN.STEEM can be found as a top tab on OpenLedger marked "SEND". Just fill out the appropriate fields and select OPEN.STEEM from the drop down in the "Amount" field.
[UPDATE] You can now also use openledgers withdraw function from the "Deposit/Withdraw" tab. Just select OPEN.STEEM from the drop down, click Withdraw Now and fill in the fields. In this case the "To" field is the destination STEEM account and no memo is necessary.
I recommend trying with a small amount first before transferring large amounts to ensure everything is working and you've enter everything correctly.
The process should only take a few seconds to complete.
If anything goes wrong just contact someone on the STEEM slack channel or on the BitSharesTalk Forum someone there will help you sort it out.