HI Everson
Open Task uses default configurations in SWFVISU which are often only appropriate for Desktop use. In the My Inbox configuration for each task you can specify which tasks should show on which devices by assigning the Consumer Type to the task.
If you want to use Open Task on mobile you must assign an appropriate option for mobile use. Eg. Point the task to UI5_BSP or use an intent based navigation as per SAP Note http://service.sap.com/sap/support/notes/2274940
Alternatively you can configure Decision Keys against the Task in the Gateway & add the code to perform the action in the matching badi /IWWRK/BADI_WF_BEFORE_UPD_IB
A 3rd option is to use the app's extensibility options to create your own buttons...but I'd suggest you try the other 2 options first.
HOpe that helps
rgds
Jocelyn