Your CPU must support overrides (not all CPUs support them, check your PLC manual)
Your DirectSOFT link's protocol must support overrides. K-Sequence supports overrides, but DirectNet does not. Make sure your PLC serial port is configured for K-Sequence and make sure your Link is using K-Sequence. If you are using an Ethernet link, make sure you are using K-Sequence or ECOM protocol (not the ECOM module, but ECOM protocol under the protocol tab).