How to activate kerning in LibreOffice?

Kerning is activated by using the Format->Character menu, selecting the Position tab and using the Pair kerning checkbox in the Spacing section.

Do a "Select All" before you turn kerning on. Otherwise it will apply only to a local selection, possible the current paragraph.

(I am using up to date libreoffice on a Mac, but this should still apply.)

There are two types of kerning out there. The old style involves pairs of letters while the new style, GPOS kerning, supports pairing groups of letters. Adobe FontCreator and others now use GPOS and GSUB rather than the "legacy" kern table method.

The Linux Libertine font project (Wikipedia's font of choice) discusses kerning:

Nearly any good font contains kerning information, but how much time and effort the designer had spent on this feature is quite different. Until the late 1990s, the kerning information was defined in a long table pair by pair. Since OpenType there is the GPOS Kerning method, which allows a group by group approach. This is a much more elegant solution, because modern fonts with more than 2000 characters would otherwise contain thousands of entries.

The problem is that GPOS support is very rare. OpenOffice bug 31764 and LibreOffice bug 46055 both request GPOS kerning support and neither seems to have much attention.

2017 Update: LO resolved this in 2016/11. GPOS will be enabled by default in LibreOffice ~5.3.