If you want to connect your Google Plus business account to your WordPress.com site:
1) Go to Google Plus at https://plus.google.com/ and log in to the account of your Google+ business page.
2) Then in WordPress.com, go to your Dashboard under Settings > Sharing.
3) Beside Google+, click Connect and it should take you to a page on Google where you can select from your different Google+ accounts. Just select your Google+ business account.
If you want to connect your personal Google+ account to WordPress.com instead, you can follow the steps outlined here: http://en.support.wordpress.com/google-plus-profile/