To install IMP and its prerequisites

You can conveniently download IMP and its prerequisites and install them into Eclipse using the Eclipse Update Manager to access the respective update sites.

To set up Eclipse to access these update sites

Bring up Eclipse.

Navigate:
Select:
Push:
This brings up the dialog "Install / Update sites to visit"; push:
This brings up a dialog in which you can enter an update-site URL as provided below.
 
Once the sites are added, you can check the boxes for the relevant sites and push "Finish" to install features from the selected sites.

LPG

The LPG update site is http://lpg.sourceforge.net/updates/

Polyglot

The Polyglot update site is http://www.cs.cornell.edu/Projects/polyglot/eclipseUpdates/

IMP

The IMP update site is http://eclipse-imp.sourceforge.net/updates/

Dependencies

You need to install the LPG and Polyglot features before you install the IMP features.  (You won't be able to install the IMP features unless the others are installed first.)

All at once

You can install IMP and its prerequisites at once by checking the boxes for all three update sites in the "Install" dialog, in which case Eclipse will sort out the dependencies and download them in the correct order.  This is convenient, too, because you can wait to restart Eclipse (which is required to put new installations into effect) until after all of the updates have been made.