This exact question was asked and answered on the Ripple forums.

It appears as if SnapSwap doesn't give appropriate instructions on what the procedure is for sending funds into a brand new Ripple account/wallet.

The general procedure is approximately this:

1. First you send then USD via your bank and provide them with your Ripple address.
2. Then they send you a small amount of XRP to initially fund your account. A brand new Ripple address cannot receive anything other than XRP so they cannot send the USD to you yet.
3. You extend a suitable amount of trust to their issuing address. This requires a small amount of XRP from step 2 be reserved.
4. They send your funds to you as a USD/SnapSwap balance in Ripple.

For future use, when you send funds into Ripple via SnapSwap to an already funded Ripple account, they should directly send the USD to your Ripple account as step 2 (assuming you have a sufficent trust line aka payment path setup).

After that you can use the USD/SnapSwap in Ripple in payments or to trade. In particular, if you want XRP you can use the "convert" page in the Ripple client to covert all or some of your USD into XRP at the best current market rate Ripple can find.