Firstly, I should point out that this forum is mainly user to user, the folks from DreamHost do post on here at times, but not too often.
Having said this, I will try my best to answer your questions.
In reply to:
How much would I have to pay if i had the Monster Package monthly pay?
The 'L3 Code Monster' package is currently on special and available for the same price as the 'L2 Sweet Dreams' package. This is about half of what the package previously cost and this lower cost is for as long as you keep the package.
Since you wish to pay monthly, the cost is $19.95 per month plus a $49.95 setup fee for a total first payment of $69.90 (and $19.95 per month after that).
In reply to:
Would i still get a discount with the code i was given on top of monster package...
Yes, assuming the promo code you were given was created to give a discount for monthly plans.
The max discount that a promo code can apply to the 'L3 Code Monster' monthly plan is $60, but the actual amount of the discount will depend on the person who created the promo code.
In reply to:
will i be using this code after i register domain name?
You enter the promo code during step 5 ('Verify Total') of the sign-up process, just before you approve payment.
I hope the above has been of some use to you.
Mark