If your source file is in the AutoCAD support file search path, you do not have to include the path name when specifying the file name. The search path is set by clicking Application menu 
Options from AutoCAD, then clicking the Files tab and selecting Support File Search Path:
For example, if you are compiling the yinyang.lsp program file that is in the AutoCADSample\VisualLISP directory, and Support File Search Path is set as indicated in the previous figure, you can issue the following command to compile the program:
(vlisp-compile 'st "yinyang.lsp")
If the AutoCADsample\visuallisp directory is not in the support file search path, you must include the entire path name when specifying the source file. For example:
(vlisp-compile 'st "c:/program files/<AutoCAD installation directory>/sample/visuallisp/yinyang.lsp")
If you omit the file extension from a file name, VLISP assumes the .lsp extension.
When specifying the file path name, replace the backslash symbol (\) you normally use for file names with either a forward slash or a double backslash, following the usual AutoCAD convention.