Dr. Yun Huang and prepared an excellent package to get started in Frequency Domain and Fatigue Analysis in LS-DYNA.
You can download the package by copying and pasting this link into a new browser window: http://ftp.lstc.com/anonymous/outgoing/huang/package/LS-DYNA_freq_domain_2014.01.16.zip 
(From FTP.LSTC.COM)