Zalando sends (new) silhouettes to the plenty FTP server so that they can be imported into your system. New silhouettes are updated weekly (Wednesdays) by us and can then be converted into attributes by you.

Use the function convert into attributes for this purpose.

If you are going through this step for the first time and it looks like this:
 

Then it is either because

1. Zalando has not yet sent any silhouettes to you OR
2. the import of your silhouettes by plentymarkets has not yet taken place.

If silhouettes are available for your system and you want to convert/update them into attributes, the view looks as follows:
 

Select the desired groups and click on convert into attributes.

TIP: only select the groups for which you actually want to start a sale, to keep the number of attributes as low as possible.

Additionally, you can find here the chapter from the manual on this topic.