Note that you can install Mighty Membership and its plugins using Mighty Assistant component: simply install Mighty Assistant, configure its Parameters, on the left side menu select Mighty Membership, check the elements you want to install and click on the 'Update/Install' button.
To install Mighty Membership manually:
1. Log into http://mightyextensions.com.
In order to log into the above mentioned website, you should be registered at mightyextensions.com. After you register, you are automatically logged into the website.
2. Navigate to Downloads-> MightyExtesnions Downloads.
3. Scroll down the Web page that is displayed after clicking MightyExtensions Downloads, and click on Mighty Membership link under the Category Index section, as shown in the following figure:
4. Scroll down the Web page that is displayed after clicking the Mighty Membership link and click the Component - Membership link (choose the one that corresponds to your php version):
5. Scroll down the Web page that is displayed after clicking the Component - Membership link. Click the com_jcs(phpx).v.x.x.x.zip link and download the Mighty Membership software:
6. Log into your Joomla back-end.
7. Navigate to Extensions-> Install/Uninstall. In the Package File text box , enter the path of the Mighty Membership software, which you have downloaded in step 5 or click the Browse button  to locate the Mighty Membership software. Click the Upload File & Install button  shown in the following figure:
Congratulation!! You have successfully installed Mighty Membership.