should I tell an interviewer that I was made a job offer.?

If this is your first interview with this company, I wouldn't mention it, at least not until the very end of the interview. It doesn't make you appear more attractive to have another company interested in you, and if you mention it too soon in the process you run the risk of having them say "Have fun at the new job". At the end of the interview you can ask when you would expect to hear something. Then when you send your follow up thank you for the interview you can mention that you do have another offer on the table but are more interested in working with them.

If I were interviewing you & you told me you had another offer & put me on the spot to make a decision before I'm ready, I would tell you to take the other job. But that's just me.

You need to find a polite way of saying that you need a response soon. Do not mention it at the beginning of the interview. If the interview goes well and you think they will offer you the job say that you have another possibility and would appreciate it if they would let you know one way or another in x amount of time.

It won't hurt to let them know...however, do not demand a response...you could say, that you have received another job offer and that they are expecting your answer back by the end of the following week (if you say that you need an answer by the end of the following week or you will take the other job, the answer you get may be "We hope you are happy at the other company" - nobody likes to be rushed like that).

2 - It really doesn't make you appear any more attractive - your qualifications are what they are regardless of whether you have no other offers, one other offer or 50 other offers.

It depends what the job is. If it is a job that requires specialised knowledge that is hard to find then the company may feel a need to offer you an incentive to chose them. If it's a job flipping hamburgers there are thousands of other kids willing to do it so they would be quite happy to pass on you.

Dreadful, they will tell you to go and take it then.

Employers don't want to be blackmailed.

former Business Owner UK

Yes, if you know you don't want to work for there.