While I was interning I was offered a job. I haven't got any update from the HR further.Its been 40 days.There is registration which will start at my college for the placement. How should I ask for the offer from the company where I interned? If I register in my college I have to attend the interview process in my college and I can't opt out.
You should write back to the person who made the offer (mentor/manager), and keep the company HR in cc for this.
Make them aware that your campus placements are due in time [X], and that unless you receive an offer letter by date [Y], as per college policies, you will have to sit for placements, and attend interviews.
Further, tell them that you would really like to join them as you feel [insert your reasons here].
Usually, such a mail would get their attention, but if you do not hear back from them, then try to explain the situation over call to your mentor, to see if they can expedite things or not.
That said, if the company HR is being lax / makes you wait unnecessarily, it would be in your self interest to be prepared for interviews, so that you are prepared in case you do not hear back from them in time, and have to actually sit through interviews.
There are some organizations who may not give the complete offer at end of internships to prevent offer shopping, and will give it to you only at a much later date. However, that is a risk (if they decide to withdraw their verbal offers) that you need to decide upon basis the company profile. So if you do not hear back in time, being prepared for the interviews would serve your own interests.