You will need to convert your Ancestry.com family tree to a GEDCOM file.  Once you have created a GEDCOM it can be imported into RootsMagic.

For instructions on how to create a GEDCOM from an Ancestry.com family tree visit:

http://ancestry.custhelp.com/cgi-bin/ancestry.cfg/php/enduser/sab_answer.php?p_faqid=472&p_sid=pE8N76qk