Ok, another solution :-). go to your "Unknown" spouse on the Small Pedigree Screen. Highlight her, and press E (for EDIT). Add her as new individual, by giving her a first name - any name will do - Martha, Kate, etc. Note her RIN.
Next step is to locate the correct individual you wish to merge your now named "unknown" with. Press "T" for MATCH/MERGE. Select MERGE TWO INDIVIDUALS. Locate your correct spouse from the list of individuals or select her if displayed already on Small Pedigree. Next locate your now named UNKNOWN. Make sure your correct individual is on the left, then press SHIFT-! to merge.
You will have to repeat these steps for each UNKNOWN (hope you haven't got too many:-) ).