To buy Soldi they serve Monero (currently it is the only electronic currency with which you can exchange).
Go to Exchange and click to buy SDI with XMR, click add to cart,view cart and finish your order.
Open your Monero wallet, enter our XMR address as recipient, send the requested Monero, open the transactions, look at ID Transaction and copy it in the Payment ID field, enter your Monero and Soldi wallet.
To finish, click Place Order. Your order will be taken over by the system and processed in the following hours, then the sum of Soldi purchased will be sent to the address of the specified Soldi wallet.