When I import or create a new fighter, the Dual-class option is not available. Am I missing something? His Intellingence is 18 so he sould be able to Dual-Class into a mage.

in D&D 3rd edition it's Impossible to dual class
i think

he needs to be a straight fighter ie no kits, no ranger, no paladin

Even though the BGII Rules book says that you can have a kit for your first class but not your second, I couldn't dual-class with either a plain fighter or with a Kensai (which would make a great dual-class mage). Also, BGII is not 3rd Edition AD&D rules. As a matter of fact its not exactly 2nd edition either with all of the things they changed, especially the weapon proficiency bonuses. (Visit the TeamBG website if you would rather use the standard weapon proficiency bonuses, including the extra 1/2 attack for Grand Mastery.)

Your STR is prob too low
You need to have a minimum str (17 I believe)
in order to dual _from_ figther

you did make a human fighter right?cause i was able to dual class my kensai to a mage from the start.it needs to have 15 in str. i think.

15 str
17 int

does anyone notice all the "bugs" and problems with this game
jeez

After dual-classing, will the experience I obtained in my initial class also count towards the experience cap (apart from the current amount in my 2nd class)?

yes it will

You can bust a hack that will remove the cap though. The cap needs to be set to a -1, then it will allow you to keep raising levels. This may not transfer too good to a 3rd game though, so keep that in mind if you plan to do this.