Do you mean the red squiggles in the picture? You need to configure intellisense. Add the defines from the makefile and add the libnds and compiler include paths.
It has been a while since I have looked into this but it leaves you with a few issues - like attributes and stdio/stdlib calls that require some additional defines to handle calling conventions. I want to say I had to add intellisense specific code to header files using “#ifdef __INTELLISENSE__” to fix some stuff, but that is a bit ugly and there are probably better solutions.