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 choosing Tools 
Options
from the AutoCAD menu, 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 AutoCAD Sample\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 AutoCAD sample\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.