Is a canon rebel t2i compadable with a sunpak 555

SOURCE: Sunpak 2000 DZ

see if this helps:
http://www.flickr.com/groups/400d/discuss/72157603805472662/

SOURCE: Canon Macro Settings for 100mm lens and Sunpak 12R ring flash

Firstly, be sure to check flash voltage. Your 30D may accept max 6V so that your camera can be shot if the flash unit has higher voltage than 6V.
1. Use low ISO to keep low noise....around f/8 or more depends on your need and how much DOF you want.....Set the shutter speed around 1/125 to your max speed to stop any movement (your subject or camera itself shaking).
2. Above is set in Manual mode (iso100, f/8 1/125 or 1/250), then you just need to leave the flash to work in TTL mode.
3. It needs battery, the connection is for syncing to trigger and release a proper flash power (in TTL mode).
Hope this help!

SOURCE: I have a Sunpak PZ42X for Canon and a Canon T1i.

I have the same problem and i dont have a idea to solve it. Did you do something?

SOURCE: Is the Sunpak auto 144 D compatable with my Canon

This is a yes and no answer. I personally would not use his flash on any of my digital cameras due to not knowing the "Trigger Voltage" one shot out of this could fry the flash circuit board of my camera.
The flash will not TTL or i-TTL with the new flash systems nor will it synchronize properly. However yes you can use this flash as a slave (using a electronic photo cell as a trigger) it will fuse a full burst power and you need to adjust it near or away from the subject to adjust the amount of light. It can prove a real pain but it will flash just not from the camera unless you use a special WEIN accessory to restrict the trigger voltage BUT the WEIN is over $100.00 so why not just get a flash that work with the camera. You would need a Canon EX type flash something like the Canon 420 EX II will be fully compatible with the new T2i.