To add comments into the Budget grid you can add a note by clicking on a single cell, group of cells or a whole row:
Select the appropriate cells,
Click the ... (three dots menu) at the bottom right of your selection
Choose Insert Comment
Type in the comment
SAVE by clicking the floppy disk icon above the comment area
Note:
These comments will not be included in any Budget reports, to publish the notes, use the Budget Comments Report template, this can be Saved to a Report Bundle if required.