Step 1 : Launch Install New Software (3.5) or Find new software (3.4) from Help.
In Eclipse 3.5:Help-->Install New Software
In Eclipse 3.4:Help->Software Updates ->Available Software
Step 2 : Add the following site http://www.apache.org/dist/tuscany/java/sca/1.5.1/tuscany-sca-1.5.1-updatesite/
Give some time till eclipse will fetch necessary dependencies
Step 3 : Once fetching is complete , select Apache tuscany core feature
Step 4: Eclipse requires old update manager to install the plug in