Prerequisites:
-Arena Gui 3.5.
-Some version of polyglot: I use polyglot1.4.70b.exe.

Step 1: Create a directory called \engine

Step 2: Put the following files in this directory:
- polyglot1.4.70b.exe.
- All your polyglot ini files that define your polyglot books. For example:

polyglot.ini
polyglot_Alinoe.ini
polyglot_AlReem.ini
polyglot_BigBook.ini , etc....

-You fill have a different polyglot....ini file for each polyglot book that you want to use.

Step 3: Put all the polyglot books in other directory called, for example, \polyglotBooks

Alinoe.bin
AlReem.bin
BigBook.bin, etc....

Step 4: Edit every polyglotXXX.ini file following the example with Allinoe.bin: polyglotAllinoe.ini.

[PolyGlot]

EngineName="NONE"

EngineDir=..\DirectoryEngine (Whatever Junior, Komodo, Stockfish...)
EngineCommand=EnginaName.exe (Whatever Junior, Komodo, Stockfish...)
Log=false
LogFile=log.txt
Book=true
BookFile=..\_polyglotBooks\Alinoe.bin
BookTreshold=9
UseNice=true
NiceValue=20
BookRandom=true
CanPonder=false

[Engine]

*** If you want to include any special uci property of the engine you will be using.

Step 5: Do the step 4 as much as polyglot .bin files you have.

Step 6: To use polyglot with a specific ini file, polyglot1.4.70b.exe polyglotXXX.ini.

- In out example, it is polyglot1.4.70b.exe polyglotAllinoe.ini.

Step 7: Arena 3.5, go to Engine->Manage->Details->Click on New and set up your polyglot as new engine: Put the Engine Name, the directory where  polyglot1.4.70b.exe is located and Command Line Parameters, the ini file: polyglotAllinoe.ini.

- Don't forget that the ini file must be place in the same location of you polyglot1.4.70b.exe .exe.

How to use a polybook in Arena 3.5? Arena_10

Step 7: Arena 5, choose type of engine UCI as I use polyglot1.4.70b.exe that lets you use a UCI engine.

Step 8: Arena 5, Click Ok, Go to Engine->Load Engine to test that you setup is working correctly.

Congratulations, and you should be able to run your polyglot book as another engine. clap
How to use a polybook in Arena 3.5? Arena_11

Step 9: Even though you may have a main book loaded in Arena, just make sure that in Engine->Manage->Details->Books->Use Arena main books with this engine is not checked.

How to use a polybook in Arena 3.5? Arena_12