I was still researching Win 7 MEA edition from your other thread I want download links for windows7 professional OA MEA - Windows 7 Help Forums
when Derek informed me of this thread. If it indeed has MEA in the version name on the COA sticker then it might be a regional version which will normally work with the official installer ISO's, unless it pertains to a rare version with features being changed for certain reasons in certain regions. But we know most of those limited versions.
Google search finds only two results for MEA each of which was advised by MS and Dell to go ahead and Clean Reinstall with the official Windows 7 installer, provided in Clean Reinstall - Factory OEM Windows 7.
But the threads have no follow up.
As Derek advises the first thing to be certain of is that you are inputting the Product Key correctly. The only activation failure I've seen in countless OEM reinstalls we've helped with here are due to incorrect key being inputted.
If this utterly fails then what I would try to Activate Windows 7 by Phone - Windows 7 Help Forums.
If that fails I'd chat with your OEM Tech Support if only to find out what exactly MEA version is, since most will never support a Clean Reinstall due to bloatware subsidies which make up much of their consumer PC profit margin.
MS Customer Service may also be some help on what MEA version is. But we may get the answer here faster.