“Export Source..." "Decompiler ctrl+alt+," grey can't used 灰色不能用

[ephonsun]

我在以下这些版本试过，结果都一样：
eclipse 4.2.2, 4.6 win-x86_64, 4.7.2 win32,
java 1.6, jdk1.8.0_144 win-86_64, jdk1.8.0_152 win32
下载的是 eclipse-class-decompiler-update_v2.10.0,
eclipse MarketPlace 查找到的是 "eclipse class decompiler 3.0.0"，这个版本都安装试过。

请问 "Export Source..." 和 "Decompiler ctrl+alt+," 功能为什么是灰色的不能用？

Excuse me, why "Export Source..." and "Decompiler ctrl+alt+," the function is grey and can't be used?