How to add umlaut and s-set to my keyboard?.

Go to control panel, then language, then choose German.
Though you have to remember the keys where is which.

If you need umlauts only occasionaly you open MS-Word, click above Insert, then Symbol, choose the umlaut you wish and create your own shortcut (i.e for ä choose strg and a). You then can use the shortcut in almost any form.

The latter is handier for occasional use.

Assuming you're using Microsoft. Has nothing to do with Dell or eircom.