When copying from third-party software such as Microsoft Word, you may find that the text doesn't paste in with the same characteristics or treatments that they were originally, or, you find it hard to change the attributes within the Calendar Builder. This is normal behavior being all programs handle text differently.
To resolve the issue, simply select all the text in your cell, or just click anywhere in the cell, and then click the Clear Formatting icon on the right-hand side of the tool bar. This will clear all formatting/attributes and leave just basic, unformatted text. Please note, this action is NOT able to be undone, so make sure clearing the formatting is your desired result before clicking that Clear Formatting icon.