Hi Chris and welcome to the Movie Edit Pro user forums.
Normally you would get a choice of either fade or title effects, some music, or sound effects tracks. You don't have to claim them all at once so you can wait to see what you may need. Be careful of some packages supplied with MEP and additional third party packages offered as they normally come with limited consumer rights which would mean if you wanted to use them for any type of profit you should then apply for a commercial use of that content from the party that is relevant.
Did you first download and install the additional content under Help? You do not want to use your credits for anything that you might already have.
Since I have never had any credits to use with MEP or VPX, what does this look like in the Store? Can you post a screen shot? In Music Maker, credits show up at the bottom of the screen. When you click on the credits, only the items that you can purchase are shown.
As @browj2 is hinting, are you sure it is Movie Edit Pro Premium you have purchased and not Music Maker Premium? Like John I have never seen any offer for 4 free items in any version of Movie Edit Pro or Video Pro X.
I've looked at my emails and it's referred to as a 'Coupon Code' and mine was initially for, and I quote,
'This email includes your personal MAGIX Shop coupon that you can redeem at www.magix.com (specials excluded). The coupon has a value of GBP 10 | 10 EUR | AUD 15 | ZAR 100 and is valid until 1 Feb 2019'.
The upgrade to the version I have now had, 'plus 5 items of your choice from the Movie Edit Pro in-app Store.* Additional add-on programs are excluded.'
I've never bought anything from the store front but have nearly all the basic transition effects and titles on offer.
I'm now getting 'nag' emails reminding me to upgrade again even though my current licence doesn't run out until the beginning of June.