even i am facing the same problem. so i called icai
and they said to obtain a letter head from a c.a in which u have specify all ur details like registration number,center name,ur name,specimen signature,group,medium along with attestaion of the c.a. and carry the same to exam center also past recent photo on the hall ticket and sing in the blank box provided for signature and photo.
If you do not want to spend so much time running to a CA office to get the attestation done, just download the Admit card from the website and carry any ID proof to satisfy the invigilator - ID card / letter issued by ICAI, DL, PAN, Voter ID etc. to the examination hall.
it happaned to me also. i got attestion from a member of ICAI in a letter which contain my photo and signature along with the details of exam ( Inter / Final), groups which i would attend and give that letter to the head of exam center (like head mistress, if it is a school) and keep a photo copy for ur reference/evidence.
But i received a letter from Insititute which instructed me to do the above things.