the problem is that, you are not running the program from "C:\Users\Aaron\Documents\MinecraftForge\forge\mcp"
thats y CommandLine is not able to find the file specified by argument. so you should try passing the absolute path
or, modify your code a bit:
File file = new File("C:\\Users\\Aaron\\Documents\\MinecraftForge\\forge\\mcp");
for(File s : file.listFiles())
{
if(s.getName().equals("recompile.bat"))
{
DefaultExecutor executor = new DefaultExecutor();
executor.setWorkingDirectory(file);
CommandLine commandLine = new CommandLine(s.getAbsolutePath());
try
{
executor.execute(commandLine);
} catch (ExecuteException e)
{
e.printStackTrace();
} catch (IOException e)
{
e.printStackTrace();
}
}
}