To install an Origin package OPX file you can
1. drag and drop onto an open Origin 
or
2. place it in the <origin install path>\Auto Install\ folder
or
3. Use Tools > Package Manager > File > Install Package


--- Troubleshooting: ----
Tip 1.
If you are encountering a problem with Drag and Drop, copy the opx file into the Auto Install folder:
<origin exe folder>\Auto Install\
or
<user files folder>\Auto Install\

Then start Origin. 
On Vista,  start Origin as administrator by right clicking on the Origin8 icon 
and choosing 'run as administrator'.
On XP you may need to logon as an administrator.

When Origin starts, it will detect the opx file in the \Auto Install\ folder and install it.

Tip 2. If Drag and Drop presents a "forbidden" cursor instead of a plus sign, and you are on Vista, 
then open the explorer.exe program by choosing 'run as administrator', 
then browse to the file you want to Drag, and drop it on Origin.
When Origin is run as administrator, then drag and drop only works if the program you are dragging from 
(in this case explorer.exe) is also running as administrator.