Creating struct objects and puttin them into an array

You if you want to duynamically add new cards to the array, you need to wrap it in another data structure:

typedef struct  
{
    int keynr;
    int access;
    time_t lastused;
} keycard;

typedef struct 
{
    keycard *keyarray;
    size_t size;
}keystorage;

int keyCreate(keystorage *cardList, size_t keyid) 
{
    if (cardList -> keyarray == NULL || keyid + 1 > cardList -> size)
    {
        keycard *new = realloc(cardList -> keyarray, sizeof(*(cardList -> keyarray)) * (keyid + 1));

        if(!new) return -1;   //error
        cardList -> keyarray = new;
        cardList -> size = keyid + 1;
    }

    cardList -> keyarray[keyid].keynr = keyid + 100;
    cardList -> keyarray[keyid].access = 1;
    cardList -> keyarray[keyid].lastused = 0.0;
    return 0; //OK
}




int main (void) {
    keycard key;
    keystorage cards = {NULL, 0};

    keyCreate(&cards, 500);
    printf("%d", cards.keyarray[500].access);

    return 0;
}

You are passing the incorrect type to keyCreate. This function expects a pointer to keycard, but you are passing it a double pointer instead. The & means "take the address of", which turns cardList into a keyCard** type. Instead, consider the following:

void keyCreate(keycard *cardList, int keyid) {
    cardList[keyid].keynr = keyid + 100;
    cardList[keyid].access = 1;
    cardList[keyid].lastused = 0;  // time_t is most likely a signed integer
}

int main (void) {
    keycard *cardList = malloc(sizeof(keycard) * 1);
    // always check if malloc succeeds, and if it does not, handle the error somehow
    if (cardList == NULL)
    {
      fprintf(stderr, "Insufficient mem\n");
      return -1;
    }

    keyCreate(cardList, 0);
    printf("%d\n", cardList[0].access);  // the \n will flush the output and
                                         // put each item on its own line

    // cleanup when you're done, but the OS will do this for you when the
    // process exits also
    free(keyCreate);

    return 0;
}

Also, time_t is most likely a signed integer (What is ultimately a time_t typedef to?), so assigning it to 0.0 is probably not right, but you'll need to check what it typedefs to on your system.

Finally, I assume this is just an MCVE, but I'd advise against mallocing in this case. The 2 primary reasons to malloc are when you don't know how much data you'll need until runtime, or you need "a lot" of data. Neither of those are true in this case. Just from what you've presented, I'd probably do the following:

#define NUM_KEY_CARDS 1

void keyCreate(keycard *cardList, int keyid) {
        cardList[keyid].keynr = keyid + 100;
        cardList[keyid].access = 1;
        cardList[keyid].lastused = 0;  // time_t is most likely a signed integer
    }

int main (void) {
    keycard cardList[NUM_KEY_CARDS];

    for (int keyid=0; keyid<NUM_KEY_CARDS; keyid++)
    {
        keyCreate(cardList+keyid, keyid);
        // or keyCreate(&(cardList[keyid]), keyid);
        printf("%d\n", cardList[keyid].access);
    }

    return 0;
}