There can be errors/incorrect approach in two scenarios. Check both of it and figure it out
SCENARIO 1 :
Once you are running the VBoxLinuxAdditions.run or VBoxSolarisAdditions.pkg or VBoxWindowsAdditions.exe , check if all the modules are getting installed properly.
1.1.a. In case of VBoxLinuxAdditions, if
Building the VirtualBox Guest Additions kernel modules gets failed,
check the log file in /var/log/vboxadd-install.log . If the error is due to kernel version update your kernel and reboot the vm. In case of fedora,
1.1.b. yum update kernel*
1.1.c. reboot
1.2. If nothing gets failed, then all is fine. You are already having the expected kernel version
SCENARIO 2 :
If the VBoxGuestAdditions is installed (check for a folder /opt/VBoxGuestAdditions-* is present .... * represents version) you need to start it before mounting.
2.1. cd /opt/VBoxGuestAdditions-*/init && ./vboxadd start
You need to specify the user id and group id of your vm user as options to the mount command.
2.2.a. Getting uid and gid of a user:
id -u <'user'>
id -g <'user'>
2.2.b. Setting uid and gid in options of mount command:
mount -t vboxsf -o uid=x,gid=x shared_folder_name guest_folder
