Hi, I have faced the same problem. The following is the solution that worked for me.
Step 1: Shutdown Eclipse. Step 2: Extract your plug-in (Zip or rar folder) into another folder. Step 3: Navigate to the folder and copy all the contents of 'plugins' folder and paste it to Eclipse\Plugins folder Step 4: If you have another folder 'features' in your extracted folder, then copy all the contents to Eclipse\features folder
Step 5: Before re starting eclipse, clenup the contentes. for this use the following command from command line.
Assuming that you have eclipse in your C:\ folder, Issue the following command c:\Eclipse\eclipse -clean
Step 6: Eclipse will now check for the new plugins and installs them. Make sure that the plugins will be compatible with the Eclispe version. Eclipse wont recognize Incompatible versions.
To check the Installable Plugins , you can observe either new icons on the tool bar or navigate tp Window->preferences or by in the context menu.