Slitherine

Could someone clarify the Rules for applying replacements?

Do you have a specific situation you aren't clear about? Otherwise, here are the basics:

At the start of each turn, both sides get a set number of replacements (the number for each day is in the calendar).
Each player places theirs on their first impulse of the turn.
Armor replacements can be applied to armor units, and Infantry replacements can be applied to infantry, cavalry, motorized infantry and airborne.
All units--including cavalry and airborne--must be able to trace supply to receive replacements.

There's more information in the Help under reinforcements.

And if you seem stuck, be sure you are looking at the entire map, there may be units on the right side in need of replacing.