java - Eclipse enable multidex

I have this error on my Android project:

[2015-11-24 20:50:09 - Dex Loader] Unable to execute dex: method ID not in [0, 0xffff]: 65536
[2015-11-24 20:50:09 - MyApp] Conversion to Dalvik format failed: Unable to execute dex: method ID not in [0, 0xffff]: 65536

I read that, I should enable multidex but I can't.

I have this problem when I added poi-3.13-20150929.jar and poi-ooxml-3.13-20150929.jar in libs folder of my project. I added this library becouse I must write an xlsx file.

So, I would like to add multidex to resolve problem, but I don't say the way.

Please help me! Thanks!