To access DB2 remote database data source in Cognos you have to do following things on Cognos machine.

1. Install DB2 client
2. Set DB2 envoirnment variables
3. Catalog remote host and database

After above these steps, add a new DB2 data source in Cognos Configurations. Navigate to IBM Cognos Connection > Launch > IBM Cognos Administration > Configuration and click New Data Source.

I set following environment variables in /etc/profile.

 JAVA_HOME=/usr/java6_64/jre
 PATH=$JAVA_HOME/bin:$PATH:/u01/ibm_cognos/cognos/c10_64/bin:/u01/IBM/db2/V9.7/bin
 LIBPATH=/u01/ibm_cognos/cognos/c10_64/bin:/home/db2inst1/sqllib/lib32
 export JAVA_HOME
 export PATH
 export LIBPATH

DB2DIR=/u01/IBM/db2/V9.7
 export DB2DIR

INSTHOME=/home/db2inst1
 export INSTHOME

DB2INSTANCE=db2inst1
 export DB2INSTANCE