Almost every international NGO operates in Liberia. It is very possible to find voluntary unpaid work here, if you are willing to stay for a bit. Paid work is almost exclusive through international organisations. Most of these organisations require foreign staff to be recruited abroad, so it is unlikely that you would be hired just because you managed to make it to Liberia.
Liberia has very high rates of unemployment. If you are in the country for longer, try to encourage local production and employment by buying local goods and paying for services.
Liberia has just come from devastating war, so the learning curriculum is not the best at all. As a tourist personally, you can learn about many attractions in Monrovia. You can personally learn a lot about Liberia's culture, art, design,etc. It would be easy if you associated with trusted foreigners to give a personal tour and as a tourist, you can learn about this poor but interesting historical country.