[Anaraxes]

An Innate Attack is normally a ranged attack that you activate using an Attack maneuver. Normally you can't use an IA at the same as as making a normal (mundane, unenhanced) melee attack. That melee attack also requires your Attack maneuver. And you can only do one thing per turn (barring Advantages like Altered Time Rate, or magic like Great Haste).

"Link" is an Enhancement to abilities that lets you activate more than one at a time. It's meant to let you combine two or more effects that are often bought separately, but are logically part of this particular ability, for some reason. The 10% charge covers the cost for not having to take two turns to activate the two abilities (one to swing, one to launch the IA). The +20% version lets you choose whether or not to use the IA when you swing. (This might matter if there's something you didn't want to kill, or it's dark and you're trying to be sneaky, or perhaps you're hitting something that gets healed by fire, or would explode if ignited, or the IA also has some cost like using up FP. With the +10% version, you just always get fire on your sword even if you don't want it.)

All that done, you have an IA that fires whenever you do a melee attack. Note that that's really a separate attack, even though they both happen on the same turn. As separate attacks, the sword would have to penetrate DR, and the fire IA would also have to penetrate DR. So you're also likely looking for the Follow-Up "penetration modifier" (a kind of Enhancement); see B105. Making the IA a Follow-Up means that the IA doesn't make its own to-hit roll or penetrate DR. It hits if the sword attack hits, and if the sword penetrates DR, the IA doesn't also have to penetrate DR. (The simple method is to add the damage together -- but you might still have different injury modifiers on the sword (say cutting) and the IA (say burning), which means you might want to roll them separately, or use different-colored dice, or whatever.)

The Accessibility you add to the Innate Attack will limit which weapons you can add the IA to. "Any melee weapon" is less limiting than "any sword" is less limiting than "swords forged in Gondolin for the goblin wars" is less limiting than "this one particular heirloom"; the broader the category, the less the Accessibility on the IA would be worth. But if you do add an Accessibility, the IA simply can't be used at all when you attack with a different weapon. If the ability says "swords only", you can't make your mace burst into flame.

Imbuements (GURPS Power-Ups 1) were invented precisely because it's kind of awkward to build (magical) abilities that add to your (equipment-based) melee attacks. It's a slightly different way to get to the same end -- a character with the ability to make any weapon (sword, etc) they pick up burst into flame, as opposed to having a sword that does that for itself.