You're probably not in the folder where you downloaded the conversationmodder.deb file. To make sure, type the following onto xterm:
It will give a list of files and folders in ur current folder. Check if conversationmodder.deb is amoung them. If not, use the command cd to change the directory to where the file is present. ie, for example if you've downloaded conversationmodder.deb to Downloads, do the following:
then do