%else
        INSTALL_DIR := C:PROGRA~1micros~1.0VC
%endif

%if !%exists($(INSTALL_DIR))
%error blablabla...
%endif

(line 25): Test: ')' expected; got "Files". Stop.

%ifdef OMAKE_MSVC_INSTALL_DIR
    INSTALL_DIR := $(OMAKE_MSVC_INSTALL_DIR)
%echo case 1
%echo $(INSTALL_DIR)
%else
    INSTALL_DIR := C:PROGRA~1micros~1.0VC
    %echo case 2
%endif

%if !%exists($(INSTALL_DIR))
    %error blablabla
%endif