At first, Install scim package through Synaptic Package Manager.

Then download & install:

http://scim-avro.googlecode.com/files/scim-avro_0.0.2-1ubuntu9.10_i386.deb

Now, make scim your default input method:

• Open terminal and type: im-switch -c
• Watch the number beside scim, type it and press Enter.
• log off and log in again from your system

Now press Ctrl+space for switch between Bangla & English

Happy Typing...