this is a tricky one, so make sure you understand it.
first of all, all values you enter in MyEtherWallet are in wei (which is ETH+18 zeros). meaning, if you want to indicate 1 ETH, you will need to write 1+18xzero = 1,000,000,000,000,000,000.
as for the minReturn value, this indicates the minimal amount of tokens you expect to receive from the change action.
for example, consider the following scenario:
BNT is traded at 1 ETH = 50 BNT (check actual price on our BNT page)
meaning, if you wish to trade 5 ETH at this conversion rate, you would expect to receive 250 BNT (5x50=250).
as we recommend setting the minReturn to 90%-95% of the actual rate, instead of 250, you should place 225. the contract will aim to change and return 225 BNT or more (but no less). if you set the value at 250, there is a higher chance that the action will fail, resulting in losing your gas and returning no BNT.
if you require additional explanation, let me know.