Over 99,9% of the problems regarding "not received" activation emails are in fact "non-issues", so first of all: Please check (and double check) your Spam/Junk mail folder.
Ok - you checked that the activation email didn't end up in your spam filter? You might belong to the unlucky 0,01% using a not-so-nice email provider (or the activation email was rejected, because your email account was/is over quota). In this case please either consider signing up again with another email address from a nice provider with well configured servers or contact us and we'll help you asap.