In sendassettoaddress, the next parameter after the asset quantity is the amount of the native currency to send in the transaction together with the asset (to the same transaction output).
There can only be one native currency in the blockchain and it is earned by miners, based on the blockchain parameters. You can also have it earned only by the creator of the chain (and therefore the first block), after which point they sent it around as appropriate.
Yes, the tutorials don't cover the native currency because most MultiChain users have opted not to use one. But the way to use it is documented properly on the page listing all the APIs:
http://www.multichain.com/developers/json-rpc-api/