Some versions of ArcGIS have an issue with dialogs, and need a Windows registry fix.
Look under: HKEY_LOCAL_MACHINE > SOFTWARE > ESRI > ArcMap > Settings
Add the value "PreloadVBA" as a REG_DWORD and set the value to 1.
You have to re-start ArcMap, if that doesn't work you may need to reboot.

Hope this helps.