You will need to check how the cost centre field is filtered on the request, and then check Dimensions to ensure that the cost centre is available, is active and has the correct settings. To do this you will need to use both FocalPoint Administrator and Dimensions.
FocalPoint Administrator
Browse to System Options > Purchasing > Purchase Request Setup
Highlight the purchase request type being used and click Edit
Browse to the Detail tab
Change the Line Type drop down menu accordingly - if you are not selecting a stock code or price line code, this will be "Text"
Find the Cost Centre field in the list
Check what the Conditional Column, Conditional Operator and Conditional/Default Value/Order fields are set to, to identify the criteria that a cost centre must meet in order to show up and make a note of this.
Dimensions
Browse to Costing > Projects
Find the project being used on the purchase request
Click Cost Centres
Find the cost centre you wanted to use in the list - if it does not exist, it will simply need to be created, otherwise, ensure it is active and meets the criteria as noted above and if not, update accordingly