Nearly Illegible Prime Number Calculator
By starting with j = 3 and incrementing j by 2 you could skip a lot of unneeded computations because it will skip even values.  
Instead of using a float I would go with int. Both are 32-Bit and you won't need the floating point values.  
You should let your variables have some space to breathe. 

Consider  

while(primes[primes.Length-1]==0){
    if(isPrime(primes,j)){
        primes[Array.IndexOf(primes,0)]=(int)j;
    }
    j++;
}  


versus  

while (primes[primes.Length - 1] == 0)
{
    if (isPrime(primes, j))
    {
        primes[Array.IndexOf(primes, 0)] = (int)j;
    }
    j += 2;
}  


where the later is much more readable. It uses the bracing style most C# developer use as well.  
Stringilate() could be improved as well by using string.Join() which would look like so  

public static string Stringilate(int[] iterable)
{
    return string.Join(" ", iterable);
}  


and as a side note, if you need to concatenate strings in a loop its much better to use a StringBuilder instead.
You should be consitent in your coding style (public static vs. static public).
You are using braces although they might be optional. Good choice !  
Instead of a concrete type you could use var instead