What are you trying to copy / paste? What keystrokes are you trying to use to copy or paste? Are you within a register transaction where you can copy / paste?
What are your settings for Edit / Preferences / Setup / Keyboard Mapping?
Is the cursor blinking on the Memo field of the register transaction, ready to receive input? When it does, have you tried simply pressing CTRL-V to paste your number? Are you sure the process you used to copy the number from the other application worked? Or is the Copy buffer empty? (Try opening a blank new file in Notepad and see if you can copy the number into Notepad.)
What are your settings for Edit / Preferences / Setup / Keyboard Mapping? Should be "Windows Standard".