After logging in the wallet select the tab Receive Money.
For the BTC address, which you wish to add on the exchange, click on the button "Actions" in the right and select "Sign Message".
Enter the e-mail you registered with the BitStock exchange in the field "Message" in the window for signing a message and click on the button "Sign Message".
The electronic signature is shown afterwards, mark it and copy it to BitStock while adding new BTC address to your account.
You may send your questions on firstname.lastname@example.org.