If you want to install Dropbox on your Ubuntu VM, follow the instructions here:
https://www.dropbox.com/en_GB/install?os=lnx
Once the installation is over and you run dropbox for the first time, it will ask you to link it to your Dropbox account. Copy the link to do that across to your browser and use it to connect to your account.
Check that dropbox is working by typing
dropbox status
It should say it is syncing
But you probably don’t want to synchronise all your Dropbox files with your VM, so create a directory for files that you do want to sync with the Ubuntu VM on your laptop (e.g. call it ubuntusyncdir). Then on your VM issue this command to stop automatic syncing of all your Dropbox files and directories:
dropbox exclude add /home/ubuntu/Dropbox/*
Check this is working by typing this command
dropbox exclude list
And it should list all your Dropbox folders and files
Then type this command and dropbox will start syncing only the files in your chosen directory
dropbox exclude remove /home/ubuntu/Dropbox/ubuntusyncdir
Now all you need to do is use the cp or mv commands to place files into that directory and they will be synced to all your other devices. This is particularly useful if you want to look at graphics files or open a file in Excel on your laptop.
More information here: http://www.dropboxwiki.com/tips-and-tricks/using-the-official-dropbox-command-line-interface-cli#comment-1778553228
PS. Never ever remove files from your Dropbox folder on the Ubuntu VM with commands like rm -r * as this will cause the command to be synced across your Dropbox account and delete folders and files from all your Dropbox folders on whatever platform. Instead use the commands above to exclude files or directories from syncing.