In the current version you should work a little with unsafe segment.

First of all you should declare your param structure:

[StructLayout(LayoutKind.Sequential)]
public struct CK_KEY_DERIVATION_STRING_DATA
{
    public int len;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=8)]
    public byte[] data;
}

Note that I specified SizeCont=8, considering for example a DES key. If your key has different length you should specify the exact length.

Then, you should convert your structure to a byte array:

CK_KEY_DERIVATION_STRING_DATA param = new CK_KEY_DERIVATION_STRING_DATA();
param.data = new byte[]{0x10,0x10,0x10,0x10,0x10,0x10,0x10,0x10};
param.len = 8;

int len = Marshal.SizeOf(param);
IntPtr pointer = Marshal.AllocCoTaskMem(len);

Marshal.StructureToPtr(param, pointer, false);

unsafe
{
    byte* p = (byte*)pointer.ToPointer();
    byte[] b = new byte[len];
    for (int i = 0; i < len; i++)
    {
        b[i] = p[i];
    }

    Mechanism m = new Mechanism(Mechanism.CKM_XOR_BASE_AND_DATA, b);

}