SQL Agent Job - Problem trying to select user to "Run As"

I have a job that runs a script. I want the script to "RUN AS" a login account that has been added to the instance. The problem is, in the window that lists all the available user names, that user that I want to run the job is not there. In fact, a lot of users are not in the list - even though I've verified that the logins are in the instance. How do I get the login I need to appear in the list so I can get this job step to RUNA AS" with that login/user? Thank in advance to all who reply!

Hi Luis and thanks for replying! Agh - finally found out what to do! You just go to login account properties > User Mapping and map the account to the master database. The account will then shows up in the list of users. Thanks again!