Side note: the following steps working (for me) in test enviroment.
The steps that I follow for get custom activity working (selectable) are:
-> Check web.config changes and integrity structure of custom activity files (.action, .cs, etc):
<SafeControl Assembly="TestCWA1, Version=126.96.36.199, Culture=neutral, PublicKeyToken=7b0c1cea1fea1132" Namespace="TestCWA1" TypeName="*" Safe="True" SafeAgainstScript="False" />
<authorizedType Assembly="TestCWA1, Version=188.8.131.52, Culture=neutral, PublicKeyToken=7b0c1cea1fea1132" Namespace="TestCWA1" TypeName="*" Authorized="True" />
Source of the code given for this answer
Clear SharePoint Designer cache.
In my case, I create a .actions file, but I didn't added the WSS.ACTIONS schema file. The .actions file must be also contained in a SharePoint mapped folder as is described in this source.
Open SharePoint Designer and create a new Workflow and select the custom activity.
In your question, you didn't mention how you created the custom activity and (since the link of your question is broken) it's hard to continue debugging.
I followed this same steps in production enviroment, but my custom activity doesn't working for me (i.e selectable), but if anyone else have more information to add, please try expand this answer.