specified expiration

[tarzan22]

Hello, please, when trying to optimize my site gives me the following problem, someone would know how to fix?

Leverage browser caching

http://www.xxxxxxx.com/clientscript/...additional.css (not specified expiration)
http://www.xxxxxxx.com/images/buttons/collapse_40b.png (not specified expiration)
http://www.xxxxxx.com/images/icons/icon3.png (not specified expiration)

and a few more

Use vBSEO vbulletin version 4.0.2 and

Centos 5.4 y plesk