The easiest fix is to just switch to the US or ABC input source. These have the same mapping as Dutch, but with more press/hold options.

If you want to customize the Dutch input source, see this:

How to add characters to the press and hold character picker in OS X Lion?

The easiest fix is to just switch to the US or ABC input source. These have the same mapping as Dutch, but with more press/hold options.

If you want to customize the Dutch input source, see this: