You just need to provide details of the mutual fund scheme and amount that you want to invest along with your client code to your Arihant advisor and dealer and they will purchase the same in your trading account. Just like in case of shares, the mutual fund units will be allotted to your demat account. 

Whenever you want to sell certain units of mutual funds, follow the same procedure and also provide the demat instruction slip or you should have provided a POA so that Arihant can provide the units to the exchange for settlement. The amount towards sale will automatically be credited to your trading account.