OK, here's a 10 steps recapitulation for newcomers:
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0. Attach ATV2 to your Mac with USB cable.
1. Put your Apple TV 2 into DFU mode as explained
here . I'll need it for both TinyUmbrella and greenpoison JB
2. Verify that in
System profiler and copy your's ECID so you can enter it in TinyUmbrella (TU)
3. Open TU (I used ver 4.21.11). If you device was detected automatically, goto next step. If not, click on Manual ECID and paste your number. Set iDevice to AppleTV if not already set. Name it as you like.
4. Select your device from the left list and click on Save SHSH in upper right corner. In my case, TU reported "This device is NOT connected", but created SHSHs in home/.shsh without a problem
5. Run greenpoison ver. 6_1 or better (WiFi
fixed in 6_1) and click Yes, confirming you are wanting to JB your AppleTV
6. As you are already in DFU mode and USB is connected, just press Jailbreak button and wait for greenpoison to finish.
7. Confirm JB once done. This will close greenpoison App.
WAIT at least 40 sec so ATV2 is updated properly.
8. You're done. Attach to power and TV.
9. I chose inject SW from new greenpoison menu and installed Cydia. ATV rebooted.
10. Next I opened Install SW from nitoTV menu and chosen XBMC