When viewing tasks using the Tree Display it is not possible to multi-select tasks.
To multi-select tasks switch to the "List Display" by clicking on the "List Display" button on the top toolbar. The "List Display" button is positioned between the "All Descendents" button and the "Tree Display" button.
When using the "List Display" it is possible to CTRL-Click multiple tasks. It is also possible to select all tasks using "Ctrl-A" or by right clicking on any task and choosing "Select | Select All"