When choosing a fill pattern for an object, you will be presented with a window like that below.

Scroll up and down the list until you find the pattern you require, then select it and click 'select' to apply the pattern to the object.
The colour of the pattern can be change by clicking the 'Click to change colour' button (See 'Choosing a colour' for more details) and you can specify how transparent the pattern should be (i.e. how much of the background or fill colour should show through it) by moving the sliding pointer between 0 for completely opaque, to 100 for completely transparent.
Clicking 'cancel' leaves the previous pattern fill setting unchanged.