Hi . I have a second generation iPod touch version 4.1 (8B117) and the model number is MB531C . I tried to jailbreak it with Limera1n , but after it's done , there is no limerain icon to be found . Can someone give a detailed step by step on how to jailbreak this device ?

not jailbreak 4.1 limera1n,

Redsn0w 0.9.6b2 is now available for purchase. You can now Jailbreak iOS 4.1 on iPhone 4, 3GS, 3G, iPod Touch 4G, 3G, 2G, iPad 3.2.2 and Apple TV 2G.
for purchase:
http://limera1n.org/2010/11/purchase/

open up safari on your ipod touch and then go to http://www.jailbreakme.com/ when that comes up, it will ask you to jailbreak your ipod automatically. it will take a while to actually jailbreak your ipod, but it actually does work. once the jailbreak is done, cydia will come up on your spring board, and then you will have to open up cydia, and select the user option, and that will allow you to completely purchase the app. when cydia asks you to upload the database content, do so. once that happens, your ipod will be jailbroken, and you can do whatever you want

I cant give you step by step but you can look up redsnow on my friend's site and that will help you