Using my previous example, domain.com is the main domain where Wordpress and wp-hive are installed. To start a second blog using the subdomain 123.domain.com, log into your control panel and go to the subdomain manager.
Create the subdomain 123.domain.com, or select the command to EDIT the subdomain. In either case, some information should come up regarding the subdomain including a selection labeled "Path" or "Specify path for files." It is here you will select or type "domain.com/blog" where "blog" refers to the (sub)directory and/or (sub)folder where the Wordpress files are located. In my example above, the Wordpress files are in the root directory, so only "domain.com" needs to be typed in "Path." Once that is done, click on either "Create" or "Apply." You should get a message to the effect that your subdomain creation/edit was successful and that the subdomain should available once the server updates.
Once the subdomain is "live," the first time you type the subdomain in your web browser, you should get a Wordpress registration page to complete the Wordpress installation. From there, you should be able to complete the instructions in the wp-hive documentation.
One other note--I'm don't know how to "point" one top-level domain (abc123.com) to another (domain.com). The best I personally can do right now is "point" the subdomain (www.abc123.com) to the other domain, which is what I just explained.
I hope this will help you.