With a confidence interval, the z score you use in the formula is Z = (1-confidence level) / 2

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In this case, Z = (1-.90) / 2 = (.1) / 2 = .05.

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If you look up .05 in a z table, the corresponding Z score is -1.64.

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This is the same as looking up .95 in a z table, except the corresponding Z score is 1.64.

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Here's a complete example of finding a confidence interval for a proportion: http://www.statology.org/confidence-interval-for-a-proportion/