G++ error undefined reference

[jumperkid400]

this is the second time i have had this issue when passing values to and from my functions so i am clearly not getting the concept. This is really the first time i have referenced my array as a char* to get them to be strings so i guess that is the issue for me. This is how i understand the concept.

 
char* word[][4] = { { "dude" ,  "yo" , "hey" },
              { "longer" , "words" , "mores" },
              { "thisis" , "atesting" , "stingssss" } };

here is my array that i am referencing. i can do this to reference the first value.
word[0][0]
here is where i seem to be getting confused. When i pass the value to a function the types are not matching up. at first i had

void userInput (const string word);
userInput(word[difficulty][number]);
void userInput(const string &word)

to which i get this message
test.cpp:(.text+0x180): undefined reference to `userInput(std::string)'
collect2: error: ld returned 1 exit status

then i tried to make it a char

void userInput (const std::char word);
userInput(word[difficulty][number]);
void userInput(const std::char &word)

that didnt compile either. So how can i reference this array better than i have to get a result? Or how can I write the function definitions so that they will work?