Q:

Which ability should be used with the power that uses another power?

Talking about hybrid Warlock/Warlord.
Warlock has At-Will power Eldritch Blast which "counts as a ranged basic attack" and uses Charisma or Constitution vs. Reflex.
http://dnd4.wikia.com/wiki/Eldritch_blast
Warlord has many powers with "Range: Ranged weapon" such as Pin Cushion, which uses Strenght vs. AC.
http://dnd4.wikia.com/wiki/Pin_cushion
As Eldritch Blast counts as basic ranged weapon, I think I can use it to perform Pin Cushion.
But Pin Cushion uses Strenght and Eldritch Blast uses Charisma to attack roll. Which one should be used? And against which defence? AC or Reflex?
Moreover, Pin Cushion deals 2[W] + Str. modifier damage. From Eldritch Blast "[W]" will be d10, but what about added modifiers?
Bonus from Charisma (Eldritch Blast) or Strenght (Pin Cusion) applies? Or both? (As its Blast + Str.modifier = dice + Cha.modifier + Str.modifier)

A:

Eldritch Blast is not a weapon. It cannot be used as the weapon for other powers.
This nullifies most of your followup questions.
Ranged Basic Attacks are just a type of attack that anyone can perform, in the same fashion that a Melee Basic Attack can be performed by anyone, and which are commonly "granted" by other powers, or even other characters. 
By default, RBAs use Dex and MBAs use STR, but there are lots of ways to gain exceptions to those rules.  One of those ways is taking powers that "count as" or "can be used in place of" basic attacks, such as Eldritch Blast.
As a Ranged Basic Attack, Eldritch Blast can be used with any other power that specifies, "you can make a Ranged Basic Attack." It also gains bonuses from any items, feats, or class features that grant bonuses to Ranged Basic Attacks. 
This doesn't mean it's useless for you, or that your Warlock/Warlord hybrid is a bad idea.  Many, many warlord powers grant both you and an ally basic attacks.  Take, for instance, Vengeance Is Mine, which has the following effect line:

Effect: You make a basic attack against the triggering enemy, and one ally within 5 squares of you can move his or her speed and make a melee basic attack against the triggering enemy as a free action.

Because this power does not specify whether it allows you to make a Melee or a Ranged Basic Attack, you can choose between the two. And if you choose ranged, you can use Eldritch Blast for it.  Note that this power does specify that your ally must make a Melee Basic Attack.
It's also worth noting here that you can select a Melee Basic Attack called Eldritch Strike in place of Eldritch Blast, and that Eldritch Strike is often considered one of the most optimizable MBAs in the game, because it also slides its target. If you are planning on spending more time in melee than at range, it's not a bad thing to pick, especially if you are not going to focus on keeping your strength at attack-level.

