Support for the D5100 in Camera Raw (the plugin inside Photoshop that reads NEF files) began with version
6.4.1 as part of Photoshop CS5. You will have to upgrade your version of Photoshop which in your case means updating your computer.
The other option is to convert your NEF files to DNG using Adobe's DNG Converter. (Photoshop can read DNG files.) You might just make it with your old Mac because the last version of the DNG Converter that is supported for the PowerPC is version 6.4 available
here.