Tomcat 6 manager login problem

Hai Pals,

              I installed jdk6 and Tomcat6 through the "Synaptic Package Manager". I configured my environmental variables like Java_home,Path, classpath,jre_home correctly in the ~/.bashrc file. It's working good. I can able to "It Works" Message of Tomcat in the browser when I go for http://localhost:8080. But I am unable to login into the manager-webapps. I have entered the <role rolename="manager"> <user username="kk" password="kkkkk" roles="manager"> in the tomcat-users.xml file. Even though I cant login. I put the above lines in the file named(tomcat-users.xml) under conf directory. But some people says I have to look into /etc/tomcat6/tomcat-users.xml. When I tried it says I am not authorized person to edit the file. I changed the file permissions. I say symbol like "X" and a "lock" symbol on it. It means read and write protected.Please help me out of this issue. I already lost much time in this.

Regards,
karthi_slm