Hi Simy,
When you call a VI in labview the input parameters have to be defined as you would when performing a function call in C. In order to do this you must place a control or indicator on the front panel of the VI with the corresponding datatype. You must then wire the control/indicator to the connector pane for the VI (Similar to defining a function prototype).
In LabVIEW goto help>>search the labview bookshelf, LabVIEW user manual chapter 7 and look for the part on wiring the connector pane.
Hope this helps...
Tristan
Applications Engineer
National Instruments UK & Ireland