Dota 6.55/6.55b doesn't work

most common reason: file got corrupted during the transfer from one of getdota.com mirrors

You should delete the old file and try to re-download the file again in .w3x format from here or from one of official getdota mirrors

Checking MD5 sum

MD5 sum is provided to assure that the download went ok and the file isn't corrupted. The MD5 for dota 6.55b is

you can check MD5 using the "MD5 Check" freeware program found here or download it from softopedia mirror

MD5 check instruction