Update DNS records to point website traffic to a hosting provider

I have office 365 P1 plan with 50 users and office small business 1 user 

I have my email and website hosted on office 365. 

under managing my domain there re two domains 

mydomain.com

mydomain.onmicrosoft.com

when I choose mydomain.com and click manage DNS, I see one record 

Type: TXT

Host name: @

Points to: v=spf1 include:outlook.com ~all

now I want to update my DNS record to point to my new website which is hosted with a third party provider. 

I followed the steps at http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/hh335298.aspx

when I try to add a new (A) record, I can not use the host name @ as stated in the directions. the error code is that this host name is already in use. 

when I expanded the "DNS records created automatically by office 365" text, I found that there are already A and CNAME record and I can not modify them 

what do I need to do to point to my new website hosted by a third party?

thanks