Abyssinica SIL is encoded according to Unicode, so your application must support Unicode text in order to access letters other than the standard ANSI characters. Most applications now provide basic Unicode support. You will, however, need some way of entering Unicode text into your document.
Abyssinica SIL is designed to work with two advanced font technologies, Graphite and OpenType. To take advantage of the advanced typographic capabilities of this font, you must be using applications that provide an adequate level of support for Graphite or OpenType.
Other suggestions are listed here: Applications Support.
The ability to obtain the full benefits of this font is also dependent upon having means for Unicode character input.
Keyman now supports Windows, macOS, and Linux. There are now a variety of Keyman keyboards for Ethiopic languages available on the Keyman website: Keyman for Amharic (and other languages)
Various other means may be available for different operating-system platforms to create additional input methods. For instance, Windows (Vista and above) provides an Ethiopic IME.
For the Mac, version 10.2 of Mac OS X includes a mechanism for users to create custom Unicode keyboard layouts (see http://developer.apple.com/technotes/tn2002/tn2056.html).
Other suggestions are listed here: Keyboard Systems Overview.
In order to use this font with existing data that was created for use with fonts developed using custom-encoded fonts, it is necessary to re-type or convert data to produce data that is encoded in conformance with the Unicode Standard. SIL Converters and/or TECkit can be used for character encoding conversion. TECkit allows users to write their own custom conversion mappings.
Three TECkit mapping files (compiled and uncompiled) are available as a separate download SIL-Ethiopia (Latin to Fidel) Unicode mapping. They are intended for use where text has been input “phonetically” as a syllable (“be”, “ppii”, etc.) and conversion to fidel is desired (U+1264 ቤ, U+1352 ፒ, etc.).
If you used an earlier version of the Abyssinica SIL font, and used some of the Private Use Area (PUA) codepoints in your data, you may wish to use the PUA to Unicode mapping file to convert your data from PUA codepoints to Unicode 6.0.
The TECkit package is available for download from SIL’s TECkit Web site.
The SIL Converters package is available for download from SIL’s SIL Converters Web site.
Docx Converter for Legacy Ethiopic Font Encoding Systems — This tool is available (non-SIL) to assist in migrating Microsoft Word documents in pre-Unicode Ethiopic fonts into a Unicode font supporting Ethiopic script.
Other suggestions are listed here: Introduction to Text Conversion and Transliteration.