I already - put the 'devel' directory into 'drupal/modules' directory - enable the devel module on the page 'extend'
but the devel module does not appear in the sidebar, and I couldn't find it in the manage/structure/block
The devel module does not provide any UI or visible components in the sidebar; that's why you don't see anything.
Not all modules produce public-facing content, and the devel module is a good example. On the other hand, it provides some functionality that developers find useful, such as dpm() and the devel tab on entities.
What were you expecting to find in the sidebar or blocks page? Did you read somewhere that you should see something?
As per the README.txt in a Devel module, following blocks will be created:
Probably, you just haven't placed them in a desired region. On a block layout page click on Place block button next to any region - in a pop-up window all these 3 blocks mentioned above should be available.