There are two ways for Creo to handle reporting the weight of solid pipe segments, it is handled through a config.pro setting piping_material_source. PTC's documentation (here) states:
weight_length - Calculates the wet weight of the pipeline using weight/length.
material_dir - Calculates the dry weight of the pipeline using the material file. To use this option, you must set the pro_material_dir file to use.
The material_dir option links the material of the pipeline to the listed material in the pro_material_dir directory, this is fairly straightforward.
The weight_length option is a bit trickier and requires some calculations beforehand, particularly when using IPS unit system.
If you're going with this option, in Creo the mass and density values are explicitly not the weight (force) - gravity needs to be accounted for in any calculations. It is VERY important to note the difference between the Creo Parametric Default unit system and IPS unit system (shown below).
IPS is a measure of pound-force and Creo Parametric Default is a measure of pound-mass. It is important that if you are using the Creo Parametric Default unit system, the pipe text CSV files must be written as "DEFAULT" as the unit system.