There are more libraries for the ADXL345, and the instructables don't tell which one they used.
Do you know how to install a library ?
Is the folder called "libraries" with a lower case 'l' ?
If you take a look at the settings in the Arduino IDE, is your sketch location set to "My Documents > Arduino" ?
Let's do this step by step.
Step one: Which Arduino board are you using ?
Step two: Connect the ADXL345.
How is it connected ? Can you upload a photo it.
Do you want to use the I2C bus ?
For the I2C bus, connect GND to GND. Vcc and CS to Arduino pin 3.3V pin. SDA and SCL to SDA and SCL on the Arduino Board (or pin A4 and A5 on a Arduino Uno or compatible board).
Add pull-up resistors for the I2C bus. Use two 4k7 (or 10k) resistors form the SDA and SCL to 3.3V.
For a first test, you could try without those.
Step three: Run the i2c scanner.
Create a new sketch and try the i2c scanner, Arduino Playground - I2cScanner
Step four: Test the ADXL345.
Take a look at the Sparkfun page, SparkFun Triple Axis Accelerometer Breakout - ADXL345 - SEN-09836 - SparkFun Electronics
They don't provide a library for the Arduino.
They have a tutorial for the Arduino, but that is SPI mode, https://www.sparkfun.com/tutorials/240
Adafruit has a tutorial for the ADXL345: ADXL345 - Triple-Axis Accelerometer (+-2g/4g/8g/16g) w/ I2C/SPI [STEMMA QT / Qwiic] : ID 1231 : $17.50 : Adafruit Industries, Unique & fun DIY electronics and kits
However, they use a different board with a voltage regulator and more components.
But you could follow that tutorial and try to make the sensor work.
See the section about the ADXL345, Arduino Playground - HomePage
I like this one: http://bildr.org/2011/03/adxl345-arduino/
The sketch in the link you gave, doesn't even use a library. It is short and simple code, Arduino-libraries/BareBones_ADXL345.pde at master · jenschr/Arduino-libraries · GitHub
Remove libraries for the ADXL345 you don't use, they could easily get mixed up.
Step five: Think
If you have the sensor working, do you want to continue with that code, or do you want to find the library for the instructables ?
If you need to adapt the instructables sketch with your working code for the sensor, we can help.