My current character is a Marksman and he uses a Dark Bow and Silver arrows...what I want to know is should I enchant items to fortify Agility or Marksman.I want max damage since the arrows and bow are somewhat weak,so which is the better choice to fortify ?

Marksman has more effect than Agility. But neither have an effect on Bow damage about 100.

By the way, you can easily test this. Just save. Enchant, wear and look at the bow damage.

I did and saw that agility did not change the DAM of the arrows or the bow so I am a little confused why Agility is said to effect the DAM rating ?
Marksman did raise the rating of both so...

Agility does increase bow damage, but Marksman does more.

The reason that you didn't see a change in the menu is undoubtedly that you gained less than a full point of additional damage. The game uses decimals in its calculations, but it doesn't display them-- it just lops them off. So if, for instance, you were doing 7.1 points of damage, it would say "7". If you gained 0.8 points for your agility buff, the game would treat that as 7.9 points, but the menu would still just display "7".

If your an archer here are some good enchantments...

Armor: Fortify Agility, Fortify Marksmen, Fortify Sneak, and Fortify Speed.

Bows: Drain Speed (Good effect for marksmen!), Drain Health, Weakness to Magic/ Elements, Elemental Damage, Drain Agility.

I also like to carry around a hood enchanted with Night-Eye to put on in dark areas.

My archer's most valuable enchantment is detect life, and lots of it. Avoiding surprises, attacking unseen from great distances, what's not to like?

As far as a bow, use the strongest type you can find for your level. Like BlackHawkSniper, my character favors bows that include a brief but powerful drain speed enchantment.

As far as damage, don't forget to poison that first shot.