My setup may differer than yours. I am using MusicBee portable edition set up in Dropbox. I have Windows Homegroup enabled (Windows 7 & 8 only). I am also using the folder organization feature of MusicBee.
Step 1 is to clear out your current library. Your songs are set up as local paths so delete them. The prompt will give you the option to delete the tracks from your hard drive, for the love of god don't do this. Just delete them from the library.
Step 2 is to go into options and move your library to Dropbox. I set up a library folder under MusicBee portable. See this setting under the "Library" tab.
Step 3 is to change the organization path, directly under the previous setting. Change it to use a network path. Mine looks like:
Step 4, under "Monitored folders", press "Choose Folders", then "Add Network Share". Select the PC then network path to your music. I'm not actually sure if this step is necessary.
Step 5, reimport your music. It will now have the network path saved as its location, and because the library is synced in Dropbox all preferences are synced. You're done.
You need to allow read/write access via Homegroups if you haven't set that up already. You can do this by right clicking the drive in My Computer, "Properties> Sharing> Advanced Sharing> Permissions> Check all".