Which version on platform are you looking to download?
If you have any SUser ID you can download both the runtime and SDK (SUP 2.1, SUP 2.2, SMP 2.3) from Service market place.
SDK and runtime are both seperate setup in service marketplace.
you can search with 'M' Software Downloads List..
SMP 3.0 is still in Rampup and only avilable to partners participating in it.
The trail version can also be downloaded without a SUser ID but the link is down now. SAP Team might be working on this. SMP 3.0 SDK can be downloaded from SAP Mobile Platform Developer Center but a runtime is needed to work on it.