I am wondering how magic resistance works. For example, my character has 30% magic resistance. When a mage casts a spell on me, how is it determined if I resist it or not? Is it a dice roll?

I'm not sure if I understand exactly what your question is.

If you're asking what MR does, it represents the chance that a spell cast against you will fail. (Note that, even if MR doesn't stop it, your saving throws might.)

If you know that, and are just wondering specifically how the game applies the MR, I don't know. Perhaps a random number from 1-100 is selected, and if the number is less than or equal to your MR % number, the spell fails. I'm just guessing, though.

Remember also that some spells ignore Magic Resistance (like those Magic Meteors, and Lower Resistance).

For instance, when a mage casts Magic Missiles, it seems the test is made for each missile.
Viconia, who can have something like 80% MR, still typically gets hurt by at least one missile.

I would be reluctant to use the Meteors, as I don't see why they must bypass MR and not Magic Missiles or Fireballs.

MMM is one item on the cheese chart that has never bothered me. You are not sending magical energies at the foe, but rather creating little fiery rocks and throwing them--rocks that can miss, to boot. They are not really any different than sunstone sling bullets, for example. Bypassing MR makes perfect sense to me in this case.

Yes... But then again, the "rocks" are created from nothing, snap they just appear, looks magic more than material.
But I perfectly understand your argument, anyway I don't like messing with them also because they replace your weapon, and if you don't use them they stay in your hand.

Just because a say a Wyvryn is summoned through the spell Wyvryn call doesn't mean certain creatures with MR should be immune to its attack because it was created magically.
All enchanted weaponns were created "magically", not to mention the early priest spells such as shillelagh, spiritual hammer and flame blade, its not as if they are pure magical energy (such as MM) but rather real physical objects that have been 'summoned'.

And back to magic resistance....
Like what Splunge said, MR represents the chance of a spell (that does not bypass MR ) failing. This also applies to spell like abilities and some traps.

Each time a spell is cast at a creature with magic resistance the computer does a roll (2 ten sided dice with one dice representing tens and the other single units so that if a 2 was rolled on the tens dice and a 7 on the units, the total roll would be 27 - which would be less than your pc's magic resistance and would therefore have no effect)

Since you have 30% magic resistance then only about 2/3 of all the spells cast at you will work, if it was 50% then only half the spells cast at you would work.

Of course, as stated above there are some spells that bypass magic resistance, such as protection removal spells, anti-undead spells and a number of others - these will always hit, even if you had 100% MR.

I think i've heard it said somewhere that Magic Missile, being 'piercing', have a greater chance of succeeding on rolls to penetrate MR. Probably by adding something to the dice roll or somewhat.

Not in 2E, and definitely not in BG2. You might have gotten this impression because, since the spell fires up to 5 missiles, if your opponent has (say) 90% MR, there's quite a high chance that at least one of the missiles will hit him.

yeah, that might be it