I have downloaded the class-dump from <http://www.codethecode.com/projects/class-dump/>.
And I open class-dump with Terminal,but the result is following
Last login: Fri Apr 22 16:46:57 on ttys000
yu-brooks-imac:~ yubrook$ /Users/yubrook/Desktop/class-dump ; exit;
class-dump 3.3.3 (64 bit)
Usage: class-dump [options] 

where options are:
 -a show instance variable offsets
 -A show implementation addresses
 --arch choose a specific architecture from a universal binary (ppc, ppc64, i386, x86\_64)
 -C only display classes matching regular expression
 -f find string in method name
 -H generate header files in current directory, or directory specified with -o
 -I sort classes, categories, and protocols by inheritance (overrides -s)
 -o output directory used for -H
 -r recursively expand frameworks and fixed VM shared libraries
 -s sort classes and categories by name
 -S sort methods by name
 -t suppress header in output, for testing
 --list-arches list the arches in the file, then exit
 --sdk-root specify the SDK root path (full path, or 4.1, 4.0, 3.2, 10.6, 10.5, 3.1.3, 3.1.2, 3.1)
logout

[Process completed]

I don't understand how to run the class-dump.Please help me. Thank you!

I tap the class-dump on the command line,the result is 
yu-brooks-imac:Desktop yubrook$ class-dump
-bash: class-dump: command not found