Can anyone let me know can I get payment via Moneybookers?
Yes, you can get paid via MB, although MB is not available to certrain clients (e.g. US clients). A web search indicates that Bangladesh can use MB but not PayPal, but that some Bangladesh people have bank accounts in India, which apparently can take PayPal (though I'm not sure how legal that is).
I am a free member. What is the difference between free member and paid member?
I don't think becoming a paid member will do you much good. There are 816 translators here who claim English-Bengali as a language combination, and for 394 of them it is their only language combination. When a client searches for a translator, the first page of results are all paid members (21 of them) and from the second page onwards the non-paying members are listed. If you can get just 10 KudoZ points, then you'd be in the top 5 non-paying translators in this language combination. If you pay money, you do end up on the first page of results but you'll be at the bottom of it.
Is there any milestone method in Proz?
What is a milestone method?