Download
this and install it in VMWare oder import it into VirtualBox using the import wizard.
Start it, login with login: freetz password:freetz
When you are logged in, enter svn co
http://svn.freetz.org/trunk 7140. This will download the "trunk" version of freetz into the directory 7140, you can replace 7140 with whatever you'd like.
Enter the directory (cd 7140) and enter make menuconfig. This will get you into the GUI which is in English. Selct your FritzBox Model, maybe show advanced options in case you can't find the torrent client in the packages section.
I think there is also an option to compile pretty much everything in English, so the FritzBox GUI AND the Freetz-Gui will be in english. If the torrent package is not available any more if you select English then because it's not available in English.
Then go to Packages and select the torrent client, don't ask me which one it was, you'll find it there, I'm not at my computer right now to check.
When you are done exit and when it asks if you want to save hit yes. Then enter make and wait until the compiling process is done. If there are no errors there will show up a message saying where the new images is, how big it is and how much space is left. If you used to much space then it can't be created. Then you'd need to go to patches and remove unwanted stuff.
To download the image and upload it to your computer use a ftp client or samba to "download" the image from your virtual machine. Login data is freetz and freetz again I think, if not check the topic I linked.