Library part with a user defined parameter

[suddenly]

Hi,

i'm new to creating library parts, so the problem is:

i need to create a simpe tube- or beam-like part with its length defined by user.

When i make a 'variable column' with length for different profile dimensions, i get a  'naming isssue' error message (there is no such message if the column is non-variable), and i have no clue how to solve it. If i choose to post the part in the library anyway, it is actually posted, but modifying it in the assembly works incorrectly.

'Variable cell' does nothing - when adding to assembly the table shows the default value of the cell, not modifyable (oh my).

So the point is - how do i make such a part in my own library?

I don't need any tools (like 'preparing a pipe for posting'), i really need to learn and understand the basic of the process.

*Inventor '11

I believe that the solution is simple, and i apologize for asking for it before searching through the forum.