2 Answers

Northern senators were looking to abolish slavery and admitting Missouri as a slave state was a step in the wrong direction. http://www.chacha.com/question/why-was-the-admission-of-missouri-as-a-slave-state-controversial-in-the-senate

WIKIPEDIA: Slave and Free States

In the United States of America prior to the American Civil War, a slave state was a U.S. state in which slavery was legal, whereas a free state was one in which slavery was either prohibited from its entry into the Union or eliminated over time. Slavery was one of the causes of the American Civil War and was abolished by the 13th Amendment of the United States Constitution in 1865.

The problem was keeping balance in Congress between slave and free states.  Keeping a balance assured them of never agreeing on anything, of course, much as it would be if we had a 50-50 House or Senate.  You can "Google" WIKIPEDIA: Slave and Free States, and you will get some good info from that article. 

You can also find the answer in your history book.  Look in the index for Missouri statehood, or something like that.  Try to keep up on the reading from now on...school's almost over for the summer. :D