That should be double consonants if you have been searching.
What keyboard are you using?
Thanks Peter no luck on the correct spelling either. The link you suggested is down for maintenance at the moment. I'm using a standard qwerty keyboard and selecting characters from a symbol menu (using mouse clicks) which allows some contextual ligatures (joining up some symbols correctly) but not others it would seem .... thanks for help anyway.
Why are you not using the Devanagari keyboard?
Menu > Apple > System Preferences > Languages & Text > Edit List… > check Devanagari > OK > Drag to top if you want the Mac to display in Devanagari, otherwise near the top and switch keyboards as required
Did you read the other links as well?
In the Special Characters, the Devanagari code table is 00000900 Devanagari
I'm using a standard qwerty keyboard and selecting characters from a symbol menu
To type Devanagari you use the Devangari Qwerty keyboard layout. Conjuncts are typed by using the f key between them. so "hindi" is typed h i n f d shift+i हिन्दी
The font takes care of itself.
There's no need to do anything with the Language tab of system preferences/language & text, only the Input Sources tab matters
Just ask if you need further info.