To set up your Office 365 email address with a domain at Network Solutions, you'll need to change your Network Solutions DNS settings twice. Once with a TXT verification record to verify your domain ownership, and a second time with the rest of your Office 365 DNS details to set your mail destination.
The TXT record value that you need to create is in your Office 365 account. If you need to sign back in, select Help me fix this.
Contact Network Solutions customer support at 1 (888) 642-0209 if you need additional help.