Calcium chloride will be a good addition, but alkalinity is likely to be needed to help moderate the mash pH. Given this water is so lightly mineralized, there is some room for options.
An easy way to increase alkalinity is with baking soda, but you have to be aware of where the sodium concentration is ending up. I like a bit of sodium in my beers. Keep it below about 25 ppm to avoid overdoing it. That amount of sodium limits the amount of bicarbonate you'll be able to add to about 70 ppm. That may not be enough, but then again, it might be OK depending on your grist.
The next best option for adding alkalinity is pickling lime. The problem with it is that its very strong and requires careful measurement and dosing. If you can handle that, then its a welcome option for adding alkalinity.
The last option for adding alkalinity is to use chalk. Its fickle since it doesn't dissolve fully and provide all of its alkalinity potential without going to special measures. Those measures involve introducing an acid to get the chalk to fully dissolve. A work around measure is to assume that the chalk adds only half of its potential alkalinity (bicarbonate content). That is what EZ Water assumes in its calculations. Bru'n Water assumes that the full alkalinity is added, so a brewer that doesn't perform the extra dissolving steps would add twice as much chalk as indicated in Bru'n Water's Water Adjustment sheet.
AJ's advice to not add alkalinity until you're sure the mash needs it is valid. But in this case, I can confidently state that you'll need the alkalinity. Adding the alkalinity will keep the wort pH higher. As with all mashes, the sweet spot for room-temperature mash pH is about 5.3 to 5.5. I think that aiming to the high end of the range provides a slightly fuller and rounder beer. The slightly elevated pH helps reduce fermentability slightly which I find welcome in darker beers.
Brewers will find a wide selection of balanced and proven water profiles that are based on the beer color in Bru'n Water. Those profiles have relatively low mineral content to avoid overdosing the beer and making Alka Seltzer beer.
Enjoy!
