is it ok to do C25K more than 3 times a week?

I think it depends on how you feel. I am personally too tired and sore sometimes to do C25K on back to back nights. Then again I usually go to the gym once a week right now.

I think do whatever feels good but take a break every once in a while.

I help coach at Learn to a Run group that follows the same basic program. Our runners are advised to follow the program as is. It is designed to help build your body up to being able to run. We encourage them to stick to the 3 days a week. Personally, I don't think a fourth day would hurt too much but more than that you may run into issues, especially further intothe program.
Of course, a lot of this depends on prior conditioning. This is assuming you are very new to running in general.
That said, there is no reason you can't cross train on your other days.

I did my c25k running every other day, meaning some weeks it was 3 runs, others 4. The reason they tell you to rest in between runs is so that your muscles, tendons and bones can catch up with your new activity.
There is a possibility that nothing bad might come out of running every day for you. But chances are, you'll cause yourself some injury.
Whatever you decide, just don't go being stubborn about running even when you are in pain, that's the easiest way to bench yourself for a good long time

Depends on how you fuel yourself and how much of a wimp you are at interpreting tired/achey muscles from damaged and need to rest.

Simple answer is no, unless its too much for you.

When I was using C25K, if I went over the 3 day limit, I would simply repeat the 3rd day for the following week.

For example, if I completed week 3 on Wednesday, then I would keep repeating week 3 day 3 on Thursday, Friday, Saturday, and/or Sunday. I wouldn't begin week 4 until the following Monday. I think it really helped me build stamina and endurance.

Right now I´m just building stamina and endurance. I´m usually on a treadmill with 4-6 incline and my average speed about 5km.

So from this I take it you do a slow walk on the treadmill at a reasonable elevation. Assuming that you can do that for 30 minutes at a time then you're in a reasonable place to start with a running plan.

I´m thinking about starting this plan http://www.c25k.com/c25k_treadmill.html and wondering if doing each level 5-6 times a week instead of 3 would be bad.

So the link is to one that will have you running for 30 minutes at the end, typical in most programmes. Subject to your pace that may not be 5K, but it's a good basis for that.

If you're not already running then I'd suggest that you want to stick with the plan, and run 3 times per week, subject to your programme you could avoid the second rest day that gives you and progress with a fourth session per week and move through slightly faster, or you could repeat one of the sessions in the week on a fourth day.

One of the main reasons for having the rest days is that running does give you a progressive development effect, so your muscles and connecting tissues take damage when you're training, and your rest days allow an opportunity for that damage to repair, and you come back stronger as a result. As long as you're eating properly, with enough protein predominantly, then you'll see progress. If you don't allow your body time to recover then you're storing problems for further down the line.

The first couple of weeks probably don't feel too challenging, so it makes sense to you at the moment to think about being more aggressive about it. In later weeks you'll struggle if you don't give yourself time to recover. Once you're running consistently then you can afford to increase your running days and put in more challenging sessions, but for now just concentrate on getting to 30 minutes.

That does not mean that you don't do anything on the off days, I'd suggest at least some bodyweight resistance training, as that helps develop the core and improves running form. As an alternative a rowing workout would be good for you as well. If anything you can use the C25K audio on the rowing machine for a good workout.