The central location for all SAP downloads is in Service Market Place. Specifically, the SAP Support Portal. Once there, you will need to traverse the menu options as appropriate. For example, since Arthur Kirchner is just looking for the SDK and not the PC client, he should navigate:
Intallations and Upgrages (left hand side) --> A-Z Index --> "S" --> SDK for ASE
Since Mr. Splinter is looking for ASE PC Client - which is shipped as part of the ASE image, he needs to traverse:
Intallations and Upgrages (left hand side) --> A-Z Index --> "A" --> SAP Adaptive Server Enterprise
(note: SAP Adaptive Server Enterprise link only has 15. & 16.0....for older versions use SY ASE under the same "A" menu)
....of course, both components are part of the ASE Express Edition or ASE Developers edition - I will ask one of the website managers to add a link to those downloads on the forum