Is it possible that I can merge jforum's user's table with my application's user's table? If yes, can you please guide me where to do the changes?

I don't know what "merging users table" really means for you.
If you remain keeping two users tables, one for your application, the other for JForum, then SSO would do the trick.
You can use your application user's username to login JForum by implementing JForum SSO.
See this post http://jforum.andowson.com/posts/list/35.page as a reference.

If you just want to keep one users table for both your application and JForum, then you can keep jforum_users table
and add new columns into jforum_users for your applications.

I dont want to keep two user's table. I want jforum to use my existing user's table. I have changed all queries, is there anything else left to change?

You can try to run it and check the log for error message.
If nothing went wrong, then you have succeeded.

I can not find the log file in tmp directory

1.Check if you have a log4j.xml in your jforum/WEB-INF/classes directory. If no, copy from jforum/WEB-INF/log4j_template.xml.
2.Restart JForum and check if /tmp has jforum.log there.