The thread Windows 7 & Photoshop CS5.1 - “Fonts missing” issue - I have the font! contains the answer:

The PSD file comes from another operating system, where the font-names are just a bit different from the Windows fonts that came with the document. Therefore Photoshop finds them as missing, although they seem to exist in the Windows Font viewer.

The above thread suggests several solutions :

1. Ask for better fonts that suite the document better
2. Add font-substitution in the registry at
   HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes.
   This way you don't modify the document and you can return it unharmed to the sender.
3. Change the fonts in the PSD file to the fonts that you have installed.
   To change a font in multiple layers at once see Editing multiple layers of type together.
   However, the changed document may not work on the Mac.

The thread Windows 7 & Photoshop CS5.1 - “Fonts missing” issue - I have the font! contains the answer:

The PSD file comes from another operating system, where the font-names are just a bit different from the Windows fonts that came with the document. Therefore Photoshop finds them as missing, although they seem to exist in the Windows Font viewer.

The above thread suggests several solutions :

1. Ask for better fonts that suite the document better
2. Add font-substitution in the registry at
   HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes.
   This way you don't modify the document and you can return it unharmed to the sender.
3. Change the fonts in the PSD file to the fonts that you have installed.
   To change a font in multiple layers at once see Editing multiple layers of type together.
   However, the changed document may not work on the Mac.