I bought the Mac version of EU3: Complete off Steam as well, and my code was also too long to register here. I solved this by logging into a Windows version of Steam; when you go to My Games for EU3:C, a different CD-Key is listed which is the correct length.
I mailed with Paradox Interactive support and for Hearts of Iron III and EU III from Steam you have to just try entering as many digits as you can ignoring the rest. So if your key from steam looks like
12345-12345-12345-12345-12345-1234 than enter it like this
1234-5123-4512-3451-2345-1234
Also Europa Universallis III from Steam must registered as "pure" Europa Universalis III not as EU3 Complete.
I bought the Mac version of EU3: Complete off Steam as well, and my code was also too long to register here. I solved this by logging into a Windows version of Steam; when you go to My Games for EU3:C, a different CD-Key is listed which is the correct length.