An LDR can be connected to any of the I/O pads (A,B,C or D). If you are using Sparkle commands, though, the Sparkle needs to be connected to D so then one leg/pad of the LDR would be connected to A,B or C and the other to a “+” pad (this could be on the battery box or the “+” output on the right-hand side of the Sparkle).
When the LDR is exposed to light it allows the current to pass through it to the I/O pad (the input will be “high”). When the LDR is covered no current flows and the input is “low”. A diagram of the set-up and example program are attached.
Let me know if any of this doesn’t make sense!