Got to Image > Options > Plugin Paths, you should see the following.
If the plugin paths are not there, you can edit the CADlink.ini and find the section headed [Adobe Plugins], you can paste the paths from here:
[Adobe Plugins]
; - Number Directories represents the current number
; of user defined directories containing adobe plugins
; - 0-10 represents the directory paths
Number Directories=7
0=C:\CADlink\SignLab 9.1\plugins
1=C:\CADlink\SignLab 9.1\plugins\Redfield
2=C:\CADlink\SignLab 9.1\plugins\HarrysFilters
3=C:\CADlink\SignLab 9.1\plugins\Rosenman
4=C:\CADlink\SignLab 9.1\plugins\EyeCandy4000
5=C:\CADlink\SignLab 9.1\plugins\KnockMeOut
6=C:\CADlink\SignLab 9.1\plugins\FilterForge
Note: the Number Directories= value must be correct or you will not see all plugins!
If the Plugin paths are all present and you still do not see the plugins, look in the c:\Signlab 9.1\Plugins folder, do the plugins exist there? If not copy the content from one of the other PC's.
Close and re-open Signlab in either case after you have made the fix.