I don't know how your project is structured. If the symbol you're missing is from a compiler lib, like GCC, you need to create a new project with all the h files from the compiler install dir, and later add it (the project) as additional symbols in the project settings (look for "import symbols").

Sometimes a declaration of a symbol uses a macro that interferes with the parsing, you need to add the macro the C.tom file so the parser will disregard it. Look for token file in the SI help.