The alias associated with the PlugSound "DAT" file has been changed, or moved. Here's what you'd do to fix it.

1. In the System folder -> Extensions -> MOTU -> Plug-ins, look for a PlugSound alias (for example "PS03-Drums&Percs.dat"). Drag that alias into the TRASH.

2. Locate the original PlugSound .dat file located on the Mac HD. Single-click the file to highlight it, and choose "MAKE ALIAS" from the FILE menu.

3. Move the alias you just created into the Plug-ins folder located in the
System folder -> Extensions -> MOTU -> Plug-ins.

4. Now, change the name of the alias. Click right on the name once, and wait a second... it should become higlighted. Click your cursor so that it's at the end of the name.
Delete "alias" from the name of the file, making sure there are no spaces left over. The name of the file should end with ".dat" - again, make sure there are no spaces at the end of the name.