All plugins are either packed in an .ipk file or as just a folder..
If the plugin comes packed as an ipk file. You can install it a number of different ways:
1) Copy to USB stick, insert into the receiver then follow on screen prompts..
2) With an FTP tool like DCC or Filezilla send the ipk to /tmp folder on the receiver then install in image menu > Plugins > Software management > Install local extensions > Follow on screen instructions..
3) With an FTP tool like DCC or Filezilla send the ipk to /tmp folder then use this command in telnet/putty to install:
opkg install /tmp/*.ipk
4) With DCC connect to the receiver then click Tools > Packages > Upload Package > Select the IPK..
5) Vu+ Control Center has an ipk installer..
If the plugin comes as just a folder then FTP it to /usr/lib/enigma2/python/plugins/extensions/ ..