What Adobe product are you creating the poster in? You really shouldn't need Kuler to just create one color. I'm not sure what app you're working in but generally you can create a new swatch in the source program. Kuler is a tool for helping you create collections of colors that go well together. In many of the apps, like Ilustrator for example you can just doubleclick on a swatch to open a dialogue for altering a swatches color.
Hope that helps,