You can enable Fault tolerance on the General page of the MicroBlaze configuration dialog box.
After enabling fault tolerance in MicroBlaze, ECC is automatically enabled in the connected LMB BRAM Interface Controllers by the tools, when the system is generated. This means that nothing else needs to be configured to enable fault tolerance and minimal ECC support.
It is possible (albeit not recommended) to manually override ECC support, leaving the LMB BRAM unprotected, by disabling C_ECC in the configuration dialogs of all connected LMB BRAM Interface Controllers.
In this case, the internal MicroBlaze block RAM protection is still enabled, because fault tolerance is enabled.