8. Edit: Do NOT send any Ethereum to the contract, it will fail! Make sure that you send 0 Ether! We do NOT accept payments!

Okay sir. I tried again and it works. Thank you

Edit:
So the GAS means, the txfee?

My transaction also got failed so what is address where I have to send the gas to receive this airdrop. No information even on OP too can you please post that address which for airdrop purpose.

Yup GAS is a name for transaction fees on the ethereum platform. You can find more information about it from here: https://ethereum.stackexchange.com/questions/3/what-is-meant-by-the-term-gas

Where does this popup? On the browser? Seems very strange and unlikely to originate from MyEtherwallet website.

I do not really understand the purpose of this airdrop. Even if one cluster Token can be worth 100 Ethereum, 0.0000025 CLRT is still nothing. It will probably not even pay the registration gas :-o.

It would take a normal investor 1 Ether to receive that amount.
And depending on the total participators, it would take several months - if not years - to obtain this amount from mining.

So all by all, it's a decent amount if you look at how ClusterToken works.
Plenty of spins on the quick cash wheel on other tokens and topics, that's okay, not everybody is here for mid to long term investement, no hard feelings 

Ethereum is the gas.  The ether ecosystem is built on the concept of ether used as the exchange fuel whilst tokens provide project participation.  Buy/send a small bit of ETH to your MEW address for gas, when needed.

No surprise you don't have 250 airdrop slots filled in half a day. Rewards are way too small