The compiler invocation commands are installed to /target_dir/opt/IBM/xlc/13.1.0/bin/, where target_dir is the location of the compiler as set by the -b flag during the non-default installation.
/compiler/xlc/opt/IBM/xlc/13.1.0/bin/xlc -o hello_c hello.c
Alternatively, you can create a shortcut to the compiler invocations, as described in Creating shortcuts to the XL C invocation commands.