Click on the Edit tab and enter your username, the language you want to translate for and if you intend to translate off-line
Use Save page to save the information
When your user has been added to the group (you should get an e-mail notification), click on Start Translating in the sidebar
Enter the language you want to translate to in Language code and click on Go (You can select to suppress pages that are 100% complete.)
Choose your page to work on, from the list presented - this will take you to the tool where you choose on-line or off-line translation.
In order to reduce the risk of edit conflicts, upload your work frequently. You can continue to work on other sections from your downloaded file - import will know which sections are new.