I have been charging this mix stick for 5 hours and the red/green light is still blinking. What's wrong? I got it used but the person says it works just fine.

Here's a guide;
What do the battery lights mean on my mouse? Some keyboard and mouse models feature a battery light that shows the current status of the batteries. The following table describes the different lights and states.
Light
Device Model Status Single long green blink Rechargeable Batteries are good. This occurs when batteries are first installed or every time that the device is turned on. Slow green blink Rechargeable Device is docked in the charging station and is charging properly. Steady green Rechargeable Device is docked in the charging station and does not need additional charging. Steady green Presenter Mouse (Japanese version) Laser pointer is on. Red/green blink Wireless Device is discoverable (trying to connect to the transceiver). Slow red blink Non-rechargeable Batteries are low and need to be replaced. Slow red blink Rechargeable Batteries are low and need recharging. Fast red blink Rechargeable Device is docked in the charging station, but there is a charging error.
Reasons for errors: The wrong type of battery is installed; the battery or batteries are missing; or the battery connection is bad.
Solutions: Make sure that the correct battery or batteries are installed, and are installed correctly. If necessary, remove the batteries, wipe them with a clean cloth, and reinsert them. Steady red Presenter Mouse (Standard version) Laser pointer is on.

Click on link and download manual:
http://www.manualsbase.com/manual/218491/mp3_player/disney/mix_stick_plus_player_hannah_montana/

The notification lights indicate:
Blinking Blue - Bluetooth On
Blinking Yellow - WiFi On
Blinking Green - GSM (Phone) On
Blinking Red - Unread message
Continuous Red - Charging

Blinking green when your GSM (phone) is on is the expected behavior.

The green lights will gradually go away as your battery is being used, this is completely normal. You batteries should last for around 5-10 miles of use. If you find that your battery light falls into the red after only 2-3 hours of use and you have been charging the batteries every night for at least 8-10 hours, contact your manufacturer for service.

Conect it on a pc & it will charge.

That means that the mp3 player is charging. Once fully charged, the light will remain red.

Make sure that is turned on.(switch to the red side) Then once connected to USB port, it will start blinking red and green.Once fully charged the light will remain red. It takes about 2.5 hours to fully charge.

Charging the Player
The player needs to be charged before it can be used and every time
the battery gets low. It takes approximately 2.5 hours to charge the
Player. Once fully charged, the Player lasts approximately 8 hours.
To charge the Player:
Remove the USB cover on the Player.
Connect the Player to the PC using the Flash Plug.
The Player will begin charging. The battery indicator should begin
to flash, alternating red and green.
Once the battery is fully charged, the indicator light will stay red.
Note: If this is the first time the Player is connected, wait until
Windows detects the Player and follow the prompts to install it.

the steady red light showing when in charger shows that
it is charging. the blinking red light when taken out of
charger or turned on and off means it is not fully charged.
leave it in charger till red light turns green.