I just started homebrewing and a couple of friends told me my airlock on my plastic bucket primary should be bubbling. After 3 days of primary fementing with dry yeast, I've seen nothing, but, you can see through the bucket that there is two inches of foam on top of the wort. In addition, when you push on the lid, the odor of the wort is strong. So, I'm thinking that there isn't a good seal on the bucket. I have two questions. Is this a problem? How do I fix it?
Short answer: It's not a problem. Don't fix it.
Most of my 200-or-so fermentations have been initially in a primary pail with a loose-fitting lid, and then racked to a carboy to wait for bottling time. That way of brewing (2-vessel fermentation) used to be the norm, but keeping the wort/beer in a single vessel has become the "in" way of homebrewing recently.
I'd suggest you not worry about making your primary pail air-tight, and rack to a carboy soon after the foam drops down.
There are a couple of small things that you can try to fix it but it would be best to buy a new bucket with a proper seal from a home brew store.
One fix could be to wrap the bucket lid in electrical tape to seal it better. I have a friend who has the same issue as you and he does this which works.
Another thing depending on the lid is you could try to put some vasoline gel in the rim of the lid to help block any gaps but you may end up having to use a lot depending on how bad the leak is.
Another thing to check is if the airlock has a proper seal. You could also vasoline that.
In regards to this being a problem if your brew is fermenting then there will be CO2 escaping from the bucket from the leak in the seal, which will mean that infection wont be able to find its way inside the bucket with the pressure of the CO2.
When the fermentation stops then there will be no CO2 escaping meaning that infections can find their way into the bucket.
A way to find out when the brew has stopped fermenting would be to take hydrometer readings and compare them to what the brew company says it should be at the end of fermentation.
Although the air lock isn't always a good indicator on what is happening in the bucket, sometimes fermentation is going slowly and you don't see any bubbling, it still feels good to see the thing going crazy, and it does keep the bad bugs out. So in short, I would consider it a problem.
Make sure the air lock is filled with sanitizer to the proper level. If that's good, then I would check the seal, and if you can't figure out what's wrong, then get another lid if possible, or another bucket and lid if not. Don't do anything crazy like throw the beer because of the seal on this batch, it will more than likely be fine.
Best of luck!