Zend recognizes that Mylyn is important to Zend Studio users and thus they package Mylyn inside of their Zend Studio distributions (http://files.zend.com/help/Zend-Studio/mylyn_integration.htm). Unfortunately they have declined to become Tasktop Certified, which would provide them with the consulting they need to package Mylyn correctly, and so historically users have had trouble installing Tasktop and updating Mylyn in Zend Studio (see https://tasktop.com/bugs/show_bug.cgi?id=846 for and https://tasktop.com/bugs/show_bug.cgi?id=1546 for further information on particular versions). Please consider filing feedback with Zend directly to encourage them to update their packaging (http://www.zend.com/en/support-center/).
Depending on your version of Zend Studio, you may be able to install Tasktop into it by enabling the Eclipse Indigo update site. To do so, go to Help > Install New Software and click the Available Software Sites link near the top-right corner of the dialog. Add the Indigo site (http://download.eclipse.org/releases/indigo/) if it is not already there and make sure it is checked. This can fix a "conflicting dependency" error related to BIRT during installation.