I configure the Joomla 2.5 user profile to make the terms of service as an required field, but after the user account created, I don't see any where in the account contain the terms of service. My concern is if any user later on doesn't follow the terms of service, where I can show them the terms of service they signed.

You need to override the register page in this case.
the file is called is register.php

A tutorial on overriding can be found here: nuevvo.com/blog/item/84-k2-inheritance-sub-templating

and: getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates

Can you please tell me the code to put into register.php file to do override.

I think I misled you.
Register.php is the register form.

If the TOS link is not available then you should simply paste the link to the TOS item in there.

profile.php is the profile editing view, you might want to paste the link there as well.