Unless you have a nice comfy cushion in the bank to live off of if you lose your job, I wouldn't say a word about looking for a new job. What time you remain with the company could be hellish if you do say anything. You don't need to lie about liking every part of your job, but you don't need to go into your review being all doom and gloom either.
Ethical considerations aside, this is a tough economy right now and you are lucky to have a job. I know a lot of college educated people who are working in video stores, supermarkets, etc. and falling further and further behind because that is all they can find for work right now.
That said, if you aren't happy, you should always be looking to better your situation.
And it is much, much easier to find a job if you already have one.
Just like winning the lotto, you can't win if you don't play. You won't find a job if you don't send out resumes.
Start by putting your res up on the big boards - monster, careerbuilder, etc. That's easy and doesn't require much effort. I get 3-4 emails a week, still, about potential positions. You never know what will come in your email. You can hide your current position and contact info so your boss won't find out you are looking. You hate the town where you live? So search out jobs in other towns. Go online to the yellowpages and search for civil firms in a town you do like, and go buy a box of resume paper. You need to start somewhere.
As to your six-month review, keep it positive. Let your boss set the tone - you don't want to go in with a whine in your voice and a bad attitude. Your boss will want to know how things are going, so start with what you like about the job. Say something good, and then say "and I'd love to do more like that because that is one of the best things about my job..." Again, you don't have to lie - keep it honest, but positive. I ended up with a 10% raise after my first review and I believe it's because I agreed that I'd like more responsibility and I kept the review positive. My boss was a schmuck and the work sucked, but after that things really improved. At 11 months, my salary was up 15% from where I started and I had two people working under me by that point. You just don't know where you might go in the company if you show the interest, and you may never know if you start things out negatively.
When he gets to the part about what you don't like, stay positive no matter what! You could say something like, "well, eventually I'd like to get my P.E. and I'm a little worried about how that will work since I'm not working under a P.E. What would you suggest I could do in order to make my efforts here count as experience?" Get the conversation started on a positive bend and don't mention leaving the company - you haven't made any definite plans yet, so why open that door until you have to?
If the review goes well and you get a raise, you may decide you want to stay with the company a little longer. If it doesn't go well, you can start putting a little more effort into your job search and you will still have a job (and probably a little more money) while you do.
Your boss isn't a mind reader. Let him know what you like, enthusiastically! Let him know what you would like to see changed (no P.E.? Maybe he knows someone outside of the company who will mentor you and provide you with the supervision you need for your P.E. experience) and let him help you with that.
If he completely blows you off, despite you showing interest in a future with the company, then push your job search to the top of your priority list. But don't get fired til you find something!
Good luck with the review. Be grateful you are even getting feedback so fast - the last few places I've worked have no review system at all.
You are going to work at good places and at rotten places throughout your career - but a lot of it will depend on your own attitude.