Cannot install any package

Dear all,

Forgive me if this is really a newbie question. I've started working with Xamarin and Xamarin.Forms. Now I'd like to try installing new packages but I cannot install almost any package on any platform.

My project is a plain Xamarin.Forms one, just created with the wizard, and I am trying, for example, to install log4net or Facebook into my Forms project Packages, or into the iOS one, I get errors:

Adding 'log4net 2.0.4' to TestProject.
Could not install package 'log4net 2.0.4'. You are trying to install this package into a project that targets 'portable-net45+win+wp80+MonoTouch10+MonoAndroid10+xamarinmac20+xamarinios10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

Adding log4net...
Adding 'log4net 2.0.4' to TestProject.iOS.
Could not install package 'log4net 2.0.4'. You are trying to install this package into a project that targets 'Xamarin.iOS,Version=v1.0', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.


Adding Facebook...
Adding 'Facebook 7.0.6' to TestProject.
Could not install package 'Facebook 7.0.6'. You are trying to install this package into a project that targets 'portable-net45+win+wp80+MonoTouch10+MonoAndroid10+xamarinmac20+xamarinios10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

Adding Facebook...
Adding 'Facebook 7.0.6' to TestProject.iOS.
Could not install package 'Facebook 7.0.6'. You are trying to install this package into a project that targets 'Xamarin.iOS,Version=v1.0', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

The Xamarin version I'm using follows. Some packages I can install in the Forms project are EntityFramework, Microsoft.Net.Http (which installs also some dependencies).

Am I missing something here?

Thanks for any help!

Xamarin Studio
Version 5.10.1 (build 6)
Installation UUID: 6ca49aa0-6464-4a7d-9494-5a28f687f520
Runtime:
    Mono 4.2.1 (explicit/6dd2d0d)
    GTK+ 2.24.23 (Raleigh theme)

    Package version: 402010102

Xamarin.Profiler
Not Installed

Xamarin.Android
Version: 6.0.0.34 (Business Edition)
Android SDK: /Users/sensei/Library/Developer/Xamarin/android-sdk-mac_x86
    Supported Android versions:
        2.3   (API level 10)
        4.0.3 (API level 15)
        4.4   (API level 19)
        6.0   (API level 23)

SDK Tools Version: 24.4.1
SDK Platform Tools Version: 23.0.1
SDK Build Tools Version: 23.0.1

Java SDK: /usr
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)

Xamarin Android Player
Not Installed

Apple Developer Tools
Xcode 7.1.1 (9081)
Build 7B1005

Xamarin.Mac
Version: 2.4.0.109 (Business Edition)

Xamarin.iOS
Version: 9.2.1.54 (Business Edition)
Hash: eb4c1ef
Branch: master
Build date: 2015-12-01 02:12:30-0500

Build Information
Release ID: 510010006
Git revision: 0b60eecdb531933734519c13257d16a780274aab
Build date: 2015-12-04 20:28:20-05
Xamarin addins: 9876fd7c9837977178411ec7375b4352c0a0d6af
Build lane: monodevelop-lion-cycle6-baseline

Operating System
Mac OS X 10.11.1
Darwin Senseis-MacBook-Pro.local 15.0.0 Darwin Kernel Version 15.0.0
    Sat Sep 19 15:53:46 PDT 2015
    root:xnu-3247.10.11~1/RELEASE_X86_64 x86_64