Greetings,
I also own a LSM303D breakout. I don’t own an Arduino. Looking at the product page here, there’s no support stated for Pico or Arduino. As stated with the “Compatible with Raspberry Pi 3B+, 3, 2, B+, A+, Zero, and Zero W” line.
After asking a similar question on the forum in this post, I (with assistance from the ever helpful Hel) did manage to find this clever persons implementation on github for the LSM303D. A link to that.
Using this I was able to return meaningful data from the sensor using a Pico. It could probably be adapted for the Arduino, as I’d imagine the micro pythonic principles are not too far apart.
Another forum poster maybe able to help further for the Arduino implementation.
I hope this helps you in some way. 👍
Cheers! 😀