you don't really need to 'write' it anywhere.

steps would be

1. delete your existing format expression

2. click on your report in design mode

3. press F4 to bring up report properties

4. Under language change to hi-IN

5. right-click on your textbox

6. click text-box properties

7. click number

8. under category select number

9. check the box labeled 'use regional formatting'

10. preview report