I am currently in a semi-homebrew campaigns and my lvl 6 Firbolg fighter and his party have come across Asmodeus the king of the Hells.  I decided to strike a deal with this particular entity but my charisma is to low to be a warlock technically.  I attempted to do create a duplicate warlock class with the SPC set as constitution.  I guess I don't quite know what I'm doing with the generator.  Any help would be mostly appreciated as this would be of great help to my party as we don't have a full time spell slinger.

Create a homebrew warlock subclass and change it from CHA to CON there. 

Tried that and it wouldn't let me equip it afterwards because it 's still relying on the CHA for validity.  What I need is the Con to be that stat instead of Cha.  That's the bugger and I don't know how to work around it or I have no clue on how to use the builder properly.

Go to the character builder. At the very first page/sheet (labeled "Home") there is a slider bar under prerequisites for multiclass. Turn that off, then you'll be able to MC with warlock. Assuming you're going to be hexblade, create a subclass using that as a template. Leave everything the same except for the name (name it something different) and scroll down to where it says CHA is the primary stat and change it to CON, then save it. After you do that, add warlock and choose that subclass. If you don't see it, make sure the homebrew slider is toggled on. (The "Home" page of the character editor)

Btw, if you are doing hexblade, you'll need to manually change the bonuses for Hex Warrior. Doesn't look like there's an option to do it in the subclass editor. Same with such things as Agonizing Blast. All depends on the build as to how much can and can't be done. 

You may just want to consider different terms with Asmodeus. A warlock pact is not the only way you can make a deal with a patron.

Maybe he gives you a weapon or other magic item. The curse mechanic would fit well with this - the weapon remains bound to you whether you want it to be or not.

You can also replicate him giving you a couple spells or something with magic items. Make a wondrous item with a couple of charges and you're good to go.

This gives you and the DM a lot more leeway in how the pact works and how you might be punished if you anger your patron, rather than being bound to the mechanics of a class.

Quote from Sigvard_Vigrid >>

Go to the character builder. At the very first page/sheet (labeled "Home") there is a slider bar under prerequisites for multiclass. Turn that off, then you'll be able to MC with warlock. Assuming you're going to be hexblade, create a subclass using that as a template. Leave everything the same except for the name (name it something different) and scroll down to where it says CHA is the primary stat and change it to CON, then save it. After you do that, add warlock and choose that subclass. If you don't see it, make sure the homebrew slider is toggled on. (The "Home" page of the character editor)

 Thanks a lot for that explanation it helped loads