Using #define in defining string size C

Hope that helps In general you're taking the wrong approach here. Mixing non-data only C++ types and PInvoke will lead you to a lot of pain. Types like std::string should not be used in this scenario because the marshaller cannot correctly create them at runtime.
Instead you need to use more primitive types. In this particular scenario, based on the managed signature, the appropriate C struct is the following

struct DataLocater
{
    char filename[256];
    int sizeOfData;
    int startLocation;
    int encrypted;
};

it fixes the issue No, this won't work. The CLR builds a dispatch table for the COM interface based on the declaration. The order of the function pointers in that table is set by the order of the method definitions in your declaration. The Show() method will occupy the first slot in both cases, no trouble there. SetOptions() however is going to end up calling the 2nd one, which is actually SetFileTypes(). They have different arguments, that's going to go kaboom in a nasty way when the implementation gets garbage arguments and the stack becomes unbalanced.
You can omit any declarations from the tail end. Also note that the actual declaration of the method doesn't matter when you don't call it. Which allows you to lie and avoid having to declare their argument types. Do make sure it is obvious that the method won't actually work, name it something like void DontCallMe2().

will be helpful for those in need I'm trying to define 'N' to be the number of chars in my class, by putting #define N 10 at the beginning of my .h file, when i do that i get a lot of errors including missing (, missing ) missing ] etc.

#define N 10;
//          ^

char str[10;];
//         ^

class String
{
private:
    static const int numChars = 10;
    char str[numChars];
public:
    String();
    String(char[]);
    ~String();
};

it should still fix some issue The issue is that your reversed string is allocated on the stack rather than the heap. When your rev function returns, all of the variables in that scope will be garbage collected. You can use malloc() to allocate memory dynamically on the heap. Note that the caller is responsible for calling free() on the string to avoid a memory leak.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *rev(char *);

int main() {
    char string[100];
    printf("Enter the string to reverse: ");
    scanf("%s", string);
    char *r = rev(string);
    printf("You entered string: %s\nReversed string is: %s\n", string, r);
    free(r);
}

char *rev(char *str) {
    int i, j;
    int size = strlen(str);
    char *rev = malloc(sizeof(*rev) * (size + 1));

    for (i = size - 1, j = 0; i >= 0; i--, j++) {
        rev[j] = str[i];
    }

    rev[size] = '\0';
    return rev;
}