If you have added an PS3, you will see it in the left hand column under "devices". Click on the torrents tab and find the file you would like to transfer to your Playstation 3. Once you have found it, click and drag the file on top of the Playstation 3 tab.
You can also right click on the file and choose convert for > Playstation 3.
If you click on the conversion tab, you will see that the file you chose is currently converting.
When the file is done converting, click on the Playstation 3 tab. Insert your USB stick in to your computer. Select the file that you recently converted and click the "Transfer selected files to storage" button. This will bring up a window that allows you to select which external device you would like to transfer to. Select your USB Stick and continue. The file will then transfer to your USB stick. When it is done, you can plug that USB stick in to your Playstation and select the file for playback.
Playstation usually requires that you have a specific folder system on your USB Stick to properly read files, but there is an easy way around that. After you plug in your USB stick, navigate to "movies" or "music" on your PS3 (depending on the type of file you are trying to play). Then find your USB stick. When it is highlighted, press the triangle button and then choose the option to 'show all files'.