How good should I prepare for the interview?
Relax. Get plenty of rest the night before. Get there on time. Dress neatly. Be yourself.
On what things should I concentrate more?
An interview is all about finding out what you know, and whether you're a good fit for the organization. Joel Spolsky summed it up in four words: "Smart. Gets things done." These aren't the sorts of things you can cram for. Interviewers aren't trying to find out how many facts you can jam into your brain in the week leading up to the interview; they're trying to find out whether you know what your CV says you know (they wouldn't be interviewing you if they didn't like your CV) and whether you seem like the kind of guy who will be effective in the job they're hiring for.
That's not to say that you shouldn't prepare. Being well prepared will help you relax and make you seem (and be!) more confident in yourself and what you can do. But don't try to learn a semester's worth of algorithms or the entire Cocoa API in a few days. Instead, look back on what you've done so far in your career. Interviewers will certainly ask you about the projects that you've worked on, so review that code (if you have it); practice explaining the goals of the project(s) and how you contributed to them; think about what you learned from those projects, what worked well, and what you'd do differently the next time.
This will be my first interview as an experienced.
A year and a half is more than no experience, but it's not a lot of experience either. You'll certainly be expected to know how to work with all the standard tools like Xcode and Interface Builder, probably know something about a version control tool, etc. But you won't be expected to know everything, so don't feel badly when they ask you questions that you can't answer. It's much better to be honest about it than to try to fake it!
I haven't worked on iOS. Will that be a demerit?
Only if you're applying for a job that requires iOS development experience. And even then, MacOS and iOS development are very, very similar in many respects. If you're applying for a Mac development job, it's probably not a problem at all.
Would that be a disadvantage having unreleased projects in my resume ?
Better to have unreleased projects than no projects at all. It's nicer to have something that you can show for your efforts, but there's nothing you can do about it at this point.
Should i think of including the other projects on which I haven't
worked(but studied) into my resume, even after knowing that it is not
a good thing to do ?
No. You should be very careful to avoid any appearance of taking any kind of credit for something that you didn't do.