I have installed my extension from Extension Manager. It is shown there as installed but it is nowhere to be found in Dreamweaver.
That is a common problem when a localized Dreamweaver version is installed - dual languages.
When such is installed there are two configuration folders created - one for each language. Then When extension is installed through EM directly started the localized language configuration folder is used. But if Dreamweaver is in English the extension is not shown, because the Dreamweaver configuration folder in use is the one for English version.
Solution: To avoid the problem always call the Extension Manager Application from Dreamweaver Commands menu -> Manage Extensions
This way Extension Manager is bound to the Dreamweaver version it is called from and extension is placed in the correct configuration folder.