Is there a tool purely for editing wild pokemon? A-Map seems a bit messy and full on for a first time user.
Your best bet will be A-Map (1.92). Some information regarding the respective boxes; they are set out like this:
Grass:
20%, 20%, 10% ,10%, 10%, 5%, 5%, 4%, 4%, 1%, 1% (from slots 1 to 12).
Water:
60%, 30%, 5%, 4%, 1% (from slots 1 to 5).
Tree (for Hoenn games, when using Rock Smash etc.):
60%, 30%, 5%, 4%, 1% (from slots 1 to 5).
Fishing Rod:
OLD ROD: 70%, 30% (slots 1 & 2).
GOOD ROD: 60%, 20%, 20% (slots 3 to 5).
SUPER ROD: 40%, 30%, 15%, 10%, 5% (from slots 6 to 10).
where % is encounter percentage.