Definitely -- in fact, it's recommended that you ask questions. It shows that you're not afraid to ask for something, such as help or information, when you need it, and also that you truly are curious about the position and hopeful for the job.
An average job interview begins with a short waiting time before you enter the room you'll be interviewed in, then your potential employer / interviewer will introduce himself or herself. You'll introduce yourself, as well. Offer them a resume or any forms if you have them available at the time of introduction, then the interview begins.
They'll ask you a few questions, and in nearly every interview, they'll ask something along the lines of "Do you have any questions for me?" or "Is there anything about this position you're not sure about, or want to know more about?"
This is the proper time to ask. Asking the pay rate doesn't necessarily make it seem that you only want the job for the money -- in fact, most employers realize that if it weren't for money, candidates for a position wouldn't be there in the first place. Asking about potential salaries, if done eloquently, can show them that you pay attention to detail and feel confident that your work is worth something.
You might ask something about hours, working conditions, etc., and follow with a question along the lines of, "Also, I'm curious as to the beginning pay rate of this position, and whether there is any room for advancement?" It's a fairly common question, and the employer won't find it strange or rude of you to ask, if that's what you're worried about.
All in all, be comfortable in an interview. Don't be too casual, but feel free to speak openly with the interviewer about any questions or concerns you may have.
Good luck =)