My old atlas doesn't have a city map for Sioux City and wouldn't have it since it was built in '93 (for $4M). New ones would put in just south of I-129 (US 20) and east of I-29.
You're looking for the Line Drive sign taking you east towards the river bluff. The key is staying just south of eastbound earthwork of US 20 (which is just beyond the left field fence by about 50 yards). The park, particularly by this year, will be completely hidden by nearby development (from the south).
There's a newer second entrance is from US 75 on Singing Hills Boulevard. This is your quickest way to the park if staying at a motel on the bluff. It is also label Line Drive though there is a sign for the "Ice Center" though it's much smaller (and easier to miss) than the ballpark.
From I-29, you're more likely to see it after you've missed the exit than before. If you're on I-129 headed east, you can't help but see it at the foot of the bluffs, but you can't get there. If there are fireworks you may even see people parking along the freeway to get a free show.
As one of the more "southerly" locations in the league, this park is usually warm well into the evening. Light jacket recommended.
Day games should include sunscreen. Like Sioux Falls, there is usually a breeze to get the heat from being too overwhelming.
In case of rain, head for the car (if allowed; remember: no reentry). There isn't a lot of shelter under the stands or in the walkways.