Tzung Kuen:

> I encounter a problematic word when reading Mahasi Sayadaw's
> Visuddhi~naan.akathaa, i.e. saadhunikaneyyapuggalla
> translated by Nyanaponika Thera as 'persons of the present
> day, who had to be given guidance'. I can't find
> 'saadhunika' in any of my dictionaries. Does it mean 'of the
> present day'

Yes.

> and derive from sa-adhunaa-ika?

Almost. It should be sa + aadhunika. The aadhunika comes
from adhunaa + .nika. Taddhitas formed with the .nika suffix
require a strengthening of the vowel in the first syllable if it
is a monophthong followed by a single consonant. Thus:
a => aa; i/ii => e; u/uu => o.

adhunaa => aadhunika
abhidhamma => aabhidhammika
sakyaputta => saakyaputtika

Best wishes,
Dhammanando